Change BrowserID login path to /db/_browserid, for Sync Gateway.
Push replicator now encodes non-string filter params as JSON.

I'm not sure whether such parameters are allowed, but JSON encoding seems like the reasonable
way to send them to the remote _changes feed (as opposed to calling ObjC -description on them,
which the code used to do...)
Potential fix for issue #239.

Conflicts:
	Source/ChangeTracker/CBLChangeTracker.m
Fix for TDMultipartDownloader in case NSURLConnection gets two responses

It's possible in rare cases for the didReceiveResponse: delegate method to be called twice.
Handle this correctly by creating a new _reader within that call.
Thanks to @kjots, @chaisehocking, @drekka for tracking this down!
Speculatively fixes #236.

Conflicts:
	Source/CBLMultipartDownloader.m
	Source/TDMultipartDownloader.h