Added GitHub Enterprise support to github-* scripts

Allow users to override the GitHub API URL using the `HUBOT_GITHUB_API` variable. The default value is _https://api.github.com_, but can be overridden with _http[s]://yourdomain.com/api/v3_ in Enterprise installs.

Added support in:
* github-activity
* github-commit-link
* github-commiters
* github-issue-link
* github-issues