(#838) - added pouch.sync, db.sync, and tests

Addresses https://github.com/daleharvey/pouchdb/issues/838. Use it like this:

    db.sync(target, options)

Which is equivalent to:

    db.replicate.to(target, options);
    db.replicate.from(target, options);

Also works at the global level:

    PouchDB.sync(db1, db2, options);

...is equivalent to...

    PouchDB.replicate(db1, db2, options);
    PouchDB.replicate(db2, db1, options);

What do you think?