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 
You could do something like...

build = ProjectBuild.objects.filter(build_key=build_key).first()
return build and build.get_absolute_url()

You never need to explicitly return None in cases like this, as the default return from a Python function is None.