follow Python conventions for boolean values

True and False might be singletons and the "default" values for "boolean"
expressions, but "all" values in Python has a boolean value and should be
evaluated as such. Checking with 'is True' and 'is False' is thus confusing,
error prone and unnessarily complex.

If we anywhere rely and nullable boolean fields from the database layer and
don't want the null value to be treated as False then we should check
explicitly for null with 'is None'.

fixes #481 rhodecode emails are sent without Date header
- small code cleanup in mailing

Notification fixes
- email prefix added to .ini files
- html templates emails
- rewrote email system to use some parts from pyramid_mailer