expose close methods

There should be a `close` method exposed (for example for `MediaIoReadable`) to close the underlying connection.

The underlying `httplib2` library (https://github.com/httplib2/httplib2/blob/master/python3/httplib2/__init__.py) seems to auto-`close()` on `Exception` or on `HEAD` requests but not otherwise (i.e. not on deletion).

- related #20

Start with MediaIoReadable and related issue #20, then inspect how the httplib2 connection is held and how deletion requests are handled. Done means the relevant readable resource exposes a close method that closes the underlying connection; compare this with httplib2's documented behavior.

索引モデルが issue の本文から書いたものです。