[issue 5] Pull replication between two local DB instances fails

I've put together a CBLChangeTracker implementation that enables pull replication between two local databases.  Most of this code should look familiar, as I cribbed it from CBLRouter+Handlers and a few other classes.  I haven't tested the long poll code path yet as I was most interested in one shot replication, but I thought I'd send you the pull request anyway so you can have a look.

Let me know if you see any problems with my approach.  Thanks!