Allow the user to specify the GitHub API base url.  Fixes #42.

For GitHub Enterprise, the API url needs to be specified.  The default
will still use the public GitHub website's API.
Merge pull request #43 from nubs/github-enterprise-url

Allow the user to specify the GitHub API base url.  Fixes #42.
Fix description of method in phpdoc.
Remove extra space in phpdoc comment.
Extract Method: PromptFactory::invoke().

Most of the time the prompts are created just to be immediately invoked.
This helper method just makes that default behavior simple.