Add default user agent

Github appears to have just started enforcing the requirement for a user-agent.
http://developer.github.com/v3/#user-agent-required

so calls now fail without a user-agent (including the tests in PyGithub which make calls to github API)

I've changed github object to have a default user-agent of 'PyGithub/Python'. However, this makes a bunch of tests fail (since they detect an extra header).

Please let me know if you agree with this change and I'll fix all the failing tests.