Upload multiple files in a single rsync command.

As with gets, this makes uploads 3x faster at least.

multi_put() is now renamed simply put(), and the old single-path put() is no more.

All tests pass (including the rsync ones), plus I've tried this out on 1000 files in our real repo.
See https://github.com/foursquare/gitshed/blob/master/src/python/gitshed/content_store.py#L72