Atlassian stash plugin

This pull request includes 5 commits from pull request #52 (email for only a subset of refs) and 1 from pull request #53 (StashEnvironment), because it builds on the StashEnvironment.  The relevant commits for this pull request are the 5 at the end.

These five commits provide the source for an Atlassian Stash Plugin, which adds various GUI configuration knobs and such to the webpage for the local stash instance, as well as hooking up git_multimail to the relevant Stash specific hook.  (While Stash can use normal git post-receive hooks, there are two problems with doing so: (1) merges of pull requests are done on the server and hence bypass those, (2) those hooks don't have access to the correct username of the pusher nor the user-visible repository name that is involved.)

This pull-request is probably not quite ready to be merged as there are a few things we're fixing up locally.  We could fix those in subsequent pull requests, but the biggest question right now for me is whether the glue for hooking Stash up to git-multmail (i.e. this Atlassian Stash Plugin) even makes sense to include as part of git-multimail, or whether that glue belongs in its own separate project.