Bug 1090689 - Add MPL2.0 headers to the repo

This work fixes part3 of Bugzilla bug [1090689](https://bugzilla.mozilla.org/show_bug.cgi?id=1090689).

Nothing exciting here, this adds missing MPL2.0 headers to files in the treeherder-client repo. This repo is so tiny and most headers were already present, so there were only a couple of additions here. The tox file uses py style ` #comments`, so it received a py block.

Adding @maurodoglio for review.