Add support for GitHub Updater

Adds support for [GitHub Updater](github.com/afragen/github-updater).

The plugin URI tag points to this repo.
The branch tag is generally optional, but since we follow a git-flow process here (commit to develop, keep master as clean releases), we want to tell the updater to check that branch for changes in Version number and which archive zip to download.

Also removed the trailing slash from the Domain Path, as WordPress trims it away anyway.
Merge pull request #33 from GaryJones/develop

Add support for GitHub Updater plugin.