Restrict processing to Repo's default branch

[jag3773]

This PR restricts tX to only process the repo's default_branch. For example, if the master branch is the repo's default, updates to a test_branch will not trigger a rebuild (by checking the ref field).

Further, the html_url field in the payload is now checked to verify it came from DCS.

This change also prevents tX from processing the request if it is not a push event, by checking that the ref field exists.

There are also several documentation fixes for tX, in terms of layout and formatting.

[coveralls]

Coverage decreased (-0.07%) to 93.024% when pulling 2760a20 on restrict_to_master into 9868567 on develop.