Allow binary blobs

In order to safely transfer binary blobs/filtes, the API needs to use base64 encoding when appropriate. postBlob() however always uses "utf-8". So there needs to be either an additional argument in write() and postBlob() that specifies the encoding, or postBlob() needs to detect if the file is binary or not, and adjust the encoding automatically.

This little snippet seems to be able to detect binary data:

```
/[\x00-\x08\x0E-\x1F]/.test(data)
```

See http://stackoverflow.com/questions/1677644/detect-non-printable-characters-in-javascript

I'm currently dealing with similar issues in Prose: https://github.com/prose/prose/pull/255