Add support for filtering _changes by doc_ids, to facilitate named document replication
Fix crash handling ?key= query parameter, and implement ?keys=

Fixes #260

Conflicts:
	Source/CBL_Router.h
	Source/CBL_Router.m
Explicitly set proxy settings for TDSocketChangeTracker.

CFStream doesn't automatically look up the system HTTP proxy settings; you have to get those
settings and then set the stream to use them. :-p
Fixes #259 (I think — haven't verified this in an environment with a proxy)