Do not display an error message when subject is empty + Refactor the app to get rid of useless logic

Hey guys,

The app used to display this misleading error message when the subject line was empty (on new tickets):
![screen shot 2014-04-24 at 09 43 37](https://cloud.githubusercontent.com/assets/330573/2789741/11964898-cbb2-11e3-9edc-4ef005d49292.png)
I removed this message, the app won't try to lookup related tickets if the subject is empty. (please see the commits messages for details, fixes #7 ).
This is a heavy PR because I removed a lot of useless legacy code. Feel free to ping me if you need more info or if you need me to make more changes.

Cheers,
Pierre

cc @zendesk/quokka @maximeprades