Initialized MAIL_DEFAULT_SENDER to make Flask-Mail work correctly. (closes #41)

The setting `DEFAULT_MAIL_SENDER` is used by Coaster and Flask-Mail expects `MAIL_DEFAULT_SENDER`. Initialized `MAIL_DEFAULT_SENDER` to the value of `DEFAULT_MAIL_SENDER` to make Flask-Mail happy.
Merge pull request #42 from anandology/patch-2

Initialized MAIL_DEFAULT_SENDER to make Flask-Mail work correctly. (clos...