Add decorator for GH API calls retries

Recent usages of scc have been failing with github.GithubException 502 Server
Error. This commit adds a decorator implementing retries if a server error is
thrown. Fixes #114.
Decorate __getattr__ methods of various GH objects with retry_on_error
Decorate create_instance() method of GHManager
Merge pull request #116 from sbesson/114_retry_decorator

Add decorator for GH API calls retries