Merge pull request #112 from sbesson/111_pr_mocktesting

Add unit testing for the PullRequest object
Fix SandboxTest.setUp() in case gh.git_repo() fails with a server error

If gh.git_repo() fails due to e.g. a server error, this commit ensures
we return to the initial folder instead of staying in a deleted folder
merge master -D org -I include -E exclude: PR 115 (Fix SandboxTest.setUp() in case gh.git_repo() fails with a server error)