core: service account auth

Implement authorization using the Service Account flow.

This is functional and you authenticate using the provided .p12 key.

There has been a recent change on the format of the key, tackled in this bug on the node client.
We might want to do the same for Python, given that now when you create a new service account, the download offered is for the .json file instead of the .p12.

The 'apiui/credential' page on the developer's console offers both P12 creds (the default) and JSON creds. oauth2client 1.3 cannot process the P12 creds:

  File "gcloud/datastore/connection.py", line 56, in _request
    method='POST', headers=headers, body=data)
  File "/home/tseaver/projects/agendaless/Google/lib/python2.7/site-packages/oauth2client-1.3-py2.7.egg/oauth2client/util.py", line 129, in positional_wrapper
    return wrapped(*args, **kwargs)
  File "/home/tseaver/projects/agendaless/Google/lib/python2.7/site-packages/oauth2client-1.3-py2.7.egg/oauth2client/client.py", line 515, in new_request
    self._refresh(request_orig)
  File "/home/tseaver/projects/agendaless/Google/lib/python2.7/site-packages/oauth2client-1.3-py2.7.egg/oauth2client/client.py", line 727, in _refresh
    self._do_refresh_request(http_request)
  File "/home/tseaver/projects/agendaless/Google/lib/python2.7/site-packages/oauth2client-1.3-py2.7.egg/oauth2client/client.py", line 751, in _do_refresh_request
    body = self._generate_refresh_request_body()
  File "/home/tseaver/projects/agendaless/Google/lib/python2.7/site-packages/oauth2client-1.3-py2.7.egg/oauth2client/client.py", line 1274, in _generate_refresh_request_body
    assertion = self._generate_assertion()
  File "/home/tseaver/projects/agendaless/Google/lib/python2.7/site-packages/oauth2client-1.3-py2.7.egg/oauth2client/client.py", line 1401, in _generate_assertion
    private_key, self.private_key_password), payload)
  File "/home/tseaver/projects/agendaless/Google/lib/python2.7/site-packages/oauth2client-1.3-py2.7.egg/oauth2client/crypt.py", line 238, in from_string
    'PKCS12 format is not supported by the PyCrypto library. '
NotImplementedError: PKCS12 format is not supported by the PyCrypto library. Try converting to a "PEM" (openssl pkcs12 -in xxxxx.p12 -nodes -nocerts > privatekey.pem) or using PyOpenSSL if native code is an option.

Converting via the suggested 'openssl' command line does not fix it, either.