Made CBLRemoveFileIfExists more reliable

I was seeing warnings while deleting databases, about file-not-found
errors (which are normal.) This should keep those from being logged.
Be more careful using weak-reference ivars. Enable some warnings.
Make replicator progress logging less spammy
Added missing Apache license notices. Updated copyright statements.
Fix problems stopping a replicator

The -stop method can end up being called re-entrantly, and it wasn’t
expecting that, so Bad Stuff could happen.

Also made the CBLReplicator’s -stop method simply clear the batcher
instead of flushing it, since any queued changes in it shouldn’t be
processed anyway (they’d result in remote requests that would just
immediately be stopped.)

For #187, #191, #193
README: updated build instructions.
Better notification of database/document changes

CBLDatabaseChangeNotification now has a “changes” key in its userInfo
that contains an NSArray of CBLDatabaseChange objects. Each of those
describes a revision that was added. This is basically equivalent to an
in-process _changes feed (though without historical data.)
Fixes #174
Bumped version to 1.485
CBLManager needs to forget replications involving deleted database

When a database is deleted, CBLManager now removes all replications
involving it from its _replications array. Otherwise those replications
could be re-used if a new database with the same name is created; this
results in conflicts if the replications are persistent, since their
documents have already been deleted.
Fixes #180