expose close methods

[casperdcl]

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