Add missing content APIs

Renamed get-content to get-contents.

Added the following APIs: create-file, update-file, and delete-file.

This provides the functionality of PR 100 and 101 on mikedeboer/node-github.
Add support for url path prefixes, useful for enterprise.

This incorporates PR 106 and 119 from mikedeboer/node-github.
Version 2.4.0
Merge pull request #7 from Robert-Nelson/pull-requests

Merge outstanding Pull requests