Proposed changes for g5k client SSL mode.

These changes makes it possible to use python-grid5000 in a Grid'5000 trusted client.

The trusted client connects using a a SSL client certificate produced by Grid'5000, and push user info through the X-API-User-CN and X-Remote-Ident http headers. The values for these headers can be provided through the g5k_user argument to most function. If not the provided this value defaults to anonymous.

The code was also made compatible with python2.7.