mozharness 0.4

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I landed mozharness 0.4 today (2091ae4c21ec).

Some notable points:

• documentation;
• python virtualenv creation;
• new MercurialVCS and sourcetool, based on lib/python/util/hg and hgtool (bug 650882);
• more mixins to avoid duplication of code;
• other than bug 690911, mozharness unit tests now pass on windows and linux.

These changes are largely behind the scenes.
However, work is already under way on talosrunner (bug 640890 and bug 650887); more on that in a bit.

(What is mozharness?)