Support using `requests` in place of `httplib2`

This removes the explicit dependency on `httplib2` and now attempts to
import `requests`, falling back to `httplib2` and finally raising an
`ImportError` if neither is available. The `http_connection` argument
accepts either a `requests.Session` object or an `httplib2.Http` object.
Update docs to reflect new `requests` support
Fix wrong attribute name for requests.Response
Merge pull request #83 from evansd/support-requests-http-library

Support `requests` http library