Add refactored JGitHub package

This is a package that didn't made it into the platform.

The "current" version can be found here: https://github.com/joomla/joomla-framework/tree/staging/src/Joomla/Github

This may still need some cleanup. I'm putting it here "on the table" so interested people could get their hands and/or eyes on it.

**References:**

----

From https://github.com/joomla/joomla-framework/pull/128

This is a follow up from https://github.com/joomla/joomla-platform/pull/1820

#### Structure amd Documentation

The API is organized using the same structure as the API documentation on GitHub

**Example**

<table class="table table-bordered table-striped">
	<tr>
		<th>Package</th>
		<th>Sub Package</th>
		<th>Method</th>
		<th>Documentation</th>
	</tr>

	<tr>
		<td>Repositories</td>
		<td></td>
		<td></td>
		<td><a href="http://developer.github.com/v3/repos/">developer.github.com/v3/<b>repos</b></a></td>
	</tr>
	<tr>
		<td><code>repositories-&gt;</code></td>
		<td></td>
		<td><code>get()</code></td>
		<td><a href="http://developer.github.com/v3/repos/#get">developer.github.com/v3/<b>repos/#get</b></a></td>
	</tr>
	<tr>
		<td>Repositories</td>
		<td>Comments</td>
		<td></td>
		<td><a href="http://developer.github.com/v3/repos/comments/">developer.github.com/v3/<b>repos/comments</b></a></td>
	</tr>
	<tr>
		<td><code>repositories-&gt;</code></td>
		<td><code>comments-&gt;</code></td>
		<td><code>get()</code></td>
		<td><a href="http://developer.github.com/v3/repos/comments/#get-a-single-commit-comment">developer.github.com/v3/<b>repos/comments/#get-a-single-commit-comment</b></a>
		</td>
	</tr>
	</tr>
</table>

**Hint:** Take a look at the <a href="http://elkuku.github.io/dox/JGithub/platform/api/packages/Joomla.html">Joomla! API Documentation</a> and compare to the <a href="http://developer.github.com/v3">GitHub API Documentation</a> (the right navigation menu).

#### Backwards compatibility
All methods have been moved:

* <a href="http://elkuku.github.io/dox/JGithub/compat-chart-old-new.html">Old vs. New Methods</a>
* <a href="http://elkuku.github.io/dox/JGithub/compat-chart-new-old.html">New vs. Old Methods</a>

Proxy methods have been added to assure full backwards compatibility.

#### Additional resources

<table>
<tr>
<th>Platform</th>
<th>Framework</th>
</tr>
<tr>
<td><a href="http://elkuku.github.io/dox/JGithub/platform/api">API Doc</a></td>
<td><a href="http://elkuku.github.io/dox/JGithub/framework/api">API Doc</a></td>
</tr>
<tr>
<td><a href="http://elkuku.github.io/dox/JGithub/platform/ut-coverage/joomla_github.html">UT coverage</a></td>
<td><a href="http://elkuku.github.io/dox/JGithub/framework/ut-coverage/src_Joomla_Github.html">UT coverage</a></td>
</tr>
</table>