Added public API for local documents

This differs a bit from the CouchDB API in that
* You don't have to give the "_local/" prefix, since it's already implicit;
* No MVCC: You don't have to specify the revision ID you're replacing.
  Since the database is local to your app, and these docs aren't replicated,
  it seems like overkill to insist on this.
Fixes #6.
Restart persistent replication after change of filter etc.

If you change the .filter, .query_params, .continuous or .doc_ids property of a
persistent replication, CBLReplication will ensure that the replicator manager
restarts the replication immediately (with the new settings.)

This may be worth doing for non-persistent replications too; but this commit
doesn't cover that.
Added some more __attribute__((nonnull)) to public headers.