include signature and quoted text detection on emails

[sbenthall]

Include a method that detects email signatures and quoted text.

Look into the open source libraries released by Mailgun:

https://github.com/mailgun/talon

http://blog.mailgun.com/open-sourcing-our-email-signature-parsing-library/

[sbenthall]

We decided to roll our own solution to this. Quoted text detection has already been submitted. We've been punting for now on signature.

[sbenthall]

Quote detection is in thread.py. This can be generalized to other libraries.