Use jinja for MailNotifier emails

[bhearsum]

Buildbot logs can be extremely big. For "on-failure" e-mails it would be quite convenient to only send the last N lines of a log, which (hopefully) would include the point of failure.

[warner]

yeah, seems reasonable

[dustin]

This can be implemented with jgraff's "customMesg" support:

 http://github.com/jgraff/buildbot/commit/28b77b98aec06d7816721e368ea58a5954d2f35d

[ddunbar]

FWIW, I attached my implementation of InformativeMailNotifier?, which appends change and log information. I'm not in love with the formatting, but it is better than nothing.

[dustin]

Thanks! I think that this needs to wait on a better customMesg interface, and some kind of "library" of pre-fabricated customMesg methods.

[ddunbar]

We have the better customMesg interface, now we just need to figure out the best place to put a more configurable mailer.

[marcusl]

We could perhaps implement this using jinja templates on that branch (see [JinjaBranch]).

Those templates are quite powerful (should be possible to slice lists, etc, for the log statements), and we could pretty easily add two or three default ones in the code, plus allow the user to send in his own.