Signatures of replies sent by Google Inbox are not detected

Signatures of replies sent by Google Inbox are not detected because there are no empty line between the quatation and the signature such as the following email.

```
Hi,
This is a test reply from Google Inbox.

On Wed, May 25, 2016 at 3:13 PM wrote:

> LGTM
>
> —
> You are receiving this because you authored the thread.
> Reply to this email directly or view it on GitHub
>
--
This is signature, but it is not detected.
```

Start by reproducing the Google Inbox example from the issue and tracing the library's signature-detection logic. Done means the signature in this format is recognized without breaking detection of the quoted reply.

索引モデルが issue の本文から書いたものです。