Issue 151 alternate server

Ok.  We have an alternate server setting, proven to work using a trivial spideroak.com proxy, and persistent across sessions.  It is not yet sanity checked, though - people can set arbitrary strings - and it doesn't yet have a separate protocol and domain part.  I'll create an issue for that.

Bonus, we have persistent settings, implemented as backbone collection and models.  Settings will automatically persist, in local storage, if their retain attribute is truthy.  The server setting provides one example, and for another trivial example I've implemented persistence of the state of the "remember" switch in the add share rooms dialog.

So not only does this fix #151, it also fixes #133.