Add low-level function to list existing remotes on Git repository
Rename remotes() to get_merge_remotes() and add check in rcleanup()

Ensure merge_remotes exist locally before attempting to remove them
Decorate __getattr__ methods of various GH objects with retry_on_error
Decorate create_instance() method of GHManager
Merge pull request #115 from sbesson/sandbox_cleanup

Fix SandboxTest.setUp() in case gh.git_repo() fails with a server error
merge master -D org -I include -E exclude: PR 116 (Add decorator for GH API calls retries)
merge master -D org -I include -E exclude: PR 117 (Check for remote existence in rcleanup )