Read GitHub Enterprise URL from ENV

you must run `--login` if you use GHE:
	$ GHE_URL=<your_ghe_url> jist --login
Make code more testable
Add a spec for GitHub Enterprise support
Make `assert_requested` expected paths more exact
Store enterprise credentials in a separate file

This will let users be logged in to both enterprise github and social
github at the same time.
Use GITHUB_URL instead of GHE_URL
Merge remote-tracking branch 'kui/support_ghe_by_url'