Set default User-Agent if not otherwise specified in customHeaders

Here's a more complete patch (with tests as requested) for https://github.com/ciaranj/node-oauth/pull/139

If no User-Agent has already been passed in to via customHeaders, the default User-Agent will be set as 'Node-oauth'.  

This should satisfy GitHub's recent change (http://developer.github.com/v3/#user-agent-required) which requires a User-Agent.