Update Github status on build:created and build:queued
Merge branch 'github_status' of https://github.com/Aaron1011/travis-core into Aaron1011-github_status
GitHub status listens to job:test:queued event.
Only update on build:created for now.

Job queues are of very fine granularity, and the commit status
is already updated when the build is created, marking a pull
request as pending already.
Remove clutter from Guardfile.
Trigger build:created after the build was created.

Rather than trigger it from the object's persistence lifecycle,
it's triggered where it's created. For now, it comes with a
configuration setting to explicitly enable it to avoid possible
side effects.