Added TaggedEC2Object.remove_tags

Pull #2259 already created an add_tags method to supplement the add_tag call. There should be an analogous remove_tags method, since the DeleteTags API also supports removing multiple tags at once. An immediate use case is being able to delete all the tags of an instance in one request; with only remove_tag, this will unnecessarily require many round trips to AWS.

Tests are included in test_ec2object.py's TestRemoveTags class.

This pull request also fixes Issue #2414 by adding a local check that the values match before deleting the tags.