add a static user_agent to the api calls

Github enforces the usage of the user_agent header
in api requests. Otherwise the request is rejected.

Github recommends to either set the username or the
name of the application accessing the api:
  https://developer.github.com/v3/#user-agent-required

The user agent is now set to "rubygem-github-v3-api".
Merge pull request #14 from jdsn/user_agent_required

github enforces the user_agent header in api requests - add a static user_agent to the api calls