add bylaws

This is a short set of bylaws.  My goal here is to establish a bit of process *before* we need it, as a means of avoiding conflict and ambiguity down the road.  This covers Botherders membership and governance, as well as a fairly simple conflict resolution process.  It should be enough to make sure Buildbot carries on merrily if I'm hit by a bus or lose my marbles an start doing stupid things.

I am, of course, open to all the review and criticism you'd like to make here, but fundamentally I'm no more interested in writing what is undoubtedly n00bish legalese than you are in reading it.  So if this looks basically un-offensive to you, then let's get it merged and move on.