Look up didn't work for short samples (10->30sec)

Hi everyone,

I am trying to use pyacoustid to identify songs out of short chunks of music (typically 5 to 30 sec, kind of a Shazam use case).
I didn't succeed in identifying Daft Punk's Get Lucky by providing pyacoustid only a fragment of the song (10 sec or even 120 sec from the beggining didn't work), whereas it worked if I provide the whole song.

Here is a snippet of my code:

```
filename = 'Get_Lucky_30sec.mp3'
duration, fingerprint = acoustid.fingerprint_file(filename)
data = acoustid.lookup(API_KEY, fingerprint, duration)
```

Providing the result of fpcalc and asking for the query through the web API gives the same result.

For instance if I call fpcalc with the first 30 seconds of the song, I get the following fingerprint result:

```
./fpcalc /Users/baptistevericel/Documents/dev/tshoko/acoustid_test/var/done/GetLucky_30sec.mp3

DURATION=30
FINGERPRINT=AQAA3UmSTFKaKEkSPFuM6ULZwz9-uDpK8tBiI78k-Ec94SJCeoboFzl6dmgiHmW4cHjxo7kQnjGSXD8eMgdz9Fl-HA9G5MwsQ__wo1aCdJ0tVBuPvKGO_iGa0FlxPoHyJh-aNGgXsBZerD-eB48ELTzKJSVqSYdH5kh_GWGz4WlxSZqg_SKmkBfO_LiyBvrR12gsIj_Oo_8I_3CHU8OUHBdNFLaU48fVGvnxHOLRH_-Ms-h-yD_OXfglVFSGftBfXMejH9aHSxmmqIeeG82YC5Uf0Dy-o18STYbj5DDjo2uW49C046GDd8SzIUueI190jD904kqOB2Hko99x486LHo2T5Yj4Clq0RemR5Rua_Hhk-EekT8JZaRD74FOLPAu2H48RXnA8g10P-iF-sN3h5_BzlMdvaHERHr0O_8JMIh9Eb8iL73CUoeyOFxeaSgoR6fuh58jDcmDeo9d-4MFvzMyRc8rwHKUWNBlv_HjoFM5lPMoenEafY6OEqj9YC_j6BDN16FGOZriQalSOPtHR7Ai1bMbnQt_R7MdzEZP2dLiyB8_hh9CDH-9V9Jdw5IZTHf3xB421HPh4PBvCE78hxoV-rE0bXOjz4SW-hSJ6ToBmHWOMEk4w6AhyCBgiBVJEICAE4AIAxQQyigFgBDCEGAQEI4ILIYhChCgGAEIISIEAAgAJLIEByGgmgCIAQCOIMgAgggAjDAhAlGHEQOcAUoABZhkCBhLGDBGFCAqMAopQIB1AzDCggBIYAAQBMgJABJlxwkCQCBFgCIAIEAIIwIBBSikiCEMGMGaMEcAIoYAgkhAjICMAAWWAEQABQZxRgjDkjPDCIWSgEgIIBg
```

Trying a lookup through the web api with the following request gives an empty result:

`https://api.acoustid.org/v2/lookup?client=m_ngzjtGqA4&meta=recordings+releasegroups+compress&duration=30&fingerprint=AQAA3UmSTFKaKEkSPFuM6ULZwz9-uDpK8tBiI78k-Ec94SJCeoboFzl6dmgiHmW4cHjxo7kQnjGSXD8eMgdz9Fl-HA9G5MwsQ__wo1aCdJ0tVBuPvKGO_iGa0FlxPoHyJh-aNGgXsBZerD-eB48ELTzKJSVqSYdH5kh_GWGz4WlxSZqg_SKmkBfO_LiyBvrR12gsIj_Oo_8I_3CHU8OUHBdNFLaU48fVGvnxHOLRH_-Ms-h-yD_OXfglVFSGftBfXMejH9aHSxmmqIeeG82YC5Uf0Dy-o18STYbj5DDjo2uW49C046GDd8SzIUueI190jD904kqOB2Hko99x486LHo2T5Yj4Clq0RemR5Rua_Hhk-EekT8JZaRD74FOLPAu2H48RXnA8g10P-iF-sN3h5_BzlMdvaHERHr0O_8JMIh9Eb8iL73CUoeyOFxeaSgoR6fuh58jDcmDeo9d-4MFvzMyRc8rwHKUWNBlv_HjoFM5lPMoenEafY6OEqj9YC_j6BDN16FGOZriQalSOPtHR7Ai1bMbnQt_R7MdzEZP2dLiyB8_hh9CDH-9V9Jdw5IZTHf3xB421HPh4PBvCE78hxoV-rE0bXOjz4SW-hSJ6ToBmHWOMEk4w6AhyCBgiBVJEICAE4AIAxQQyigFgBDCEGAQEI4ILIYhChCgGAEIISIEAAgAJLIEByGgmgCIAQCOIMgAgggAjDAhAlGHEQOcAUoABZhkCBhLGDBGFCAqMAopQIB1AzDCggBIYAAQBMgJABJlxwkCQCBFgCIAIEAIIwIBBSikiCEMGMGaMEcAIoYAgkhAjICMAAWWAEQABQZxRgjDkjPDCIWSgEgIIBg`

Response:

`{"results": [], "status": "ok"}`

Is this behavior expected or do I need to use the library differently?

Thanks for your feedback

Start at the acoustid.lookup call and compare its handling of the 30-second fingerprint with the full-song case, then check the corresponding Acoustid web API request. Done means determining whether short samples are supported and recording the expected behavior or a focused change needed to make them work.

Written by the indexing model from the issue text.