BrowserID support for replicator (untested)

To use it with non persistent replications, set a TDBrowserIDAuthorizer.

With persistent replications, add a "auth" sub-object of the "source"
or "target" object (just as for OAuth) and in it put a "browserid"
object with an "assertion" property whose value is the BrowserID
assertion.

This obviously requires that the remote server have BrowserID support.
There is a CouchDB-BrowserID plugin, and IrisCouch's servers have it.
Merge branch 'master' of github.com:couchbaselabs/TouchDB-iOS into public-api

Conflicts:
	TouchDB.xcodeproj/project.pbxproj
More replicator improvements

* Added TDReplication.browserIDAssertion property
* Non-persistent TDReplications now correctly get filters/authorizers.
* Proper error reporting if a non-persistent TDReplication can't be created.
* Created a TD_DatabaseManager method to create replicators
Dumb bug fixes (TDReplication, TDBrowserIDAuthorizer)