Share map/reduce/filter/validate fns with background dbs

When a CBLManager runs a background-thread manager, the CBLDatabases in the background manager
need to have access to the callback blocks defined in the main manager, or they won't work right.
Created a CBL_Shared object to hold this shared state. Fixes #26.
Rename "BrowserID" --> "Persona"

Note that this invalidates all saved persistent replications that used Persona/BrowserID auth,
because the property in the replication doc was renamed too.
It also changes the server endpoint that the replicator hits to log in from /db/browserid to /db/persona.
Fixes #25.
Updated README
Async support!

* Added -[CBLManager asyncTellDatabaseNamed:to:].
* Added -[CBLQuery runAsync:].
* CBLLiveQuery now runs its query asynchronously in the background.
* CBLLiveQuery.rows returns nil initially (as documented), before the background query completes.
* Added -start and -stop to CBLLiveQuery.
* Fixed: CBLLiveQuery was notifying that its .rows changed even if the result of the query didn't change.
* Added CBLQueryEnumerator.error (for use with async queries.)
* kCBLStaleUpdateAfter is now implemented.
* Fixed: CBLQueryEnumerator.sequenceNumber wasn't being set.
* CBLQueryRow uses less memory (parsed versions of .key and .value are now weak.)
* More unit tests of CBLQuery.
* 'Query' logging key logs info about running async and live queries.
* Logging now writes a double-bar instead of a "|" after the timestamp if the message is from a background thread.
Fixed out-of-order change notifications

CBLReplication model objects were often getting change notifications in the wrong order,
causing confusion. This happened due to the ReplicatorManager updating a document
in response to getting a change notification for it, which then caused a re-entrant call into
the notification code, resulting in the document getting the second notification first. Yuck.

Fixes couchbaselabs/CouchChat-iOS#5
Fixed some new bugs in CBLQuery found by the static analyzer
PUT with new_edits=false should return JSON body like regular PUT

Conflicts:
	Source/CBL_Router+Handlers.m
Logic fix in CBLReplication; should take care of a reported assertion failure

Fixes #24 (I think…)
Another logic fix in CBLReplication

Fixes #24 some more.