E-mail only when build status has changed

There's a check-box on the project settings page that, when ticked, only sends e-mails:
a) on the first build :ship: 
b) the current build succeeded, but the previous failed :)
c) the current build failed, but the previous succeeded :(
The last build is based on the per-project's build IDs.

I hope the test makes sense, basically ensures that the first build fires, a matching one doesn't, but a different one does.

This helps issue #125, but only on the e-mail notifier. I guess other notifiers may not be as annoying when fired one per build :)