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.
merge master -D org -I include -E exclude: PR 115 (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)