Send an Email to the requestor on completion of a build.

New post-build task to send an Email to the build requestor.
Check for a default archive when running the generate_checksum task. Also return the build primary key from the method.

Fixes Issue #93, #108 
Do not do this, this is introducing "spaghetti dependencies".

Currently, in "master", "projects" depends on "archives" and "jenkins"

"archives" depends on credentials.

"jenkins" depends on nothing, this would introduce a dependency on
"dependencies", which is not a good thing (see the complexities of the
dependencies in hexr/certification for why).

I'd move this from the Jenkins app, possibly to the archiving, possibly to
projects.