BrowserID improvements

* Parse email address out of assertion rather than requiring it to be provided separately.
* Also get the expiration date, and don't return assertions that have expired.
Added public API for starting the REST API

Just access the .internalURL property of a TDDatabase or TDDatabaseManager to get its touchdb: URL.
Requires linking with the TouchDBListener framework, or building TDURLProtocol & TDRouter into your app.