Provide default values for regex matches in email notifications (#266)

This will make the e-mail notifications contain '[application]' when the full application details cannot be extracted from the console log. See below for an example.

Before:
> Subject: [Aborted] mozilla-central_remote:  en-US on master (20131128030201)
Mozmill mozilla-central_remote testrun for  en-US on master (20131128030201) was aborted.

After:
> Subject: [Aborted] mozilla-central_remote: [application] en-US on master (20131128030201)
Mozmill mozilla-central_remote testrun for [application] en-US on master (20131128030201) was aborted.

I'm open to suggestions for any alternatives to '[application]'