better expose popkeys and drop methods for a klepto.archive

[mmckerns]

mystic.cache.archive has read and write functions. Similar methods to "remove" a list of entries and to "delete" an archive should be added, as they compliment read and write.

"remove an entry" in a klepto.archive instance "a" should be implemented with values = a.popkeys(keys), however should potentially return a dict of popped key/value pairs.

"delete a backend" for a klepto.archive instance "a" should be implemented with a.archive.__drop__(), and should probably either return None or the contents of the archive.