Modified mail_remove_replies_after behaviour and added feature to strip gmail style replies

The config item mail_remove_replies_after is modified to accept multiple marker strings. I have tested this with Outlook style(-----Original Message-----) and Zimbra style(----- Original Message -----) reply markers.

Added a regular expression to strip gmail style replies. This cannot be modified from the configuration page. The test results are included as a screenshot.
![selection_175](https://f.cloud.github.com/assets/145110/1731880/19c1bc08-6313-11e3-8a71-2ff4a7807c33.png)

I'm not an expert in regular expressions, so I just kept both the implementations(gmail & outlook) different. May be we can store the regular expressions in mail_remove_replies_after and get rid of the hard coded gmail part.