From equal pusher

This pull request includes 5 commits from pull request #52 (email for only a subset of refs), 1 from pull request #53 (StashEnvironment), and 4 from pull request #54 (GerritEnvironment), because it builds on the Stash and Gerrit environments.  The only new relevant commit is the one at the end.

The default choice for 'From' seems suboptimal to me.  Setting it to be the person who made the push makes a lot more sense.

Possibly relevant tidbit: Within Palantir in our local customization hook that imports git_multimail.py, we actually do a replace('From: %(fromaddr)s', 'From: %(reply_to)s') on git_multimail.REVISION_HEADER_TEMPLATE as a backward compatibility hack.  (Our previous email solution that was commit based always set From=author, and although people agreed with me about the choice of From=pusher if we were starting from scratch, there was a strong desire to not force people to update their email filters in Outlook when we switched to git-multimail.)  As such, this commit was only effective within Palantir for a short time frame.  However, it is what I'd use anywhere else.