Pickling of Session and HTTPAdapter + a test

This is for issue #1088
Merge pull request #1243 from darjus-amzn/master

Pickling of Session and HTTPAdapter + a test
Fixed some pickeling issues with HTTPAdapter and Session

Added trust_env and stream to Session.__attrs__. Initialize
self._pool_connections and self._pool_maxsize in HTTPAdapter.
Merge pull request #1244 from jajadinimueter/master

Fixed further pickeling issues in Session and HTTPAdapter