Change log from GitHub Release Api

Get list of release from the GitHub Api by using the OctoKit.net library.

I had to made a modification of Octokit.net to receive the content block in Html instead of raw text GProulx/octokit.net@02a0aca62fc8b405ae670b275603713b39330f7e because using MarkdownSharp after did not gave me an interesting result. 

There is already an issue on octokit.net repo (octokit/octokit.net#593) about the need to support different body media types, I will continue to check that thread or propose an improved version of my modification so we can move to the official release in the next few weeks/months.

What's included:
* Get all releases information from the GitHub repository for each version (WE2013, WE2015)
* Cache the html result for about an hour to increase performance
* Display only the releases from the last 6 months. If there is more than that, add a link at the bottom to the full list of releases from the GitHub release page for the project.
* Display an error message with a link to the same page if there is a problem while accessing the API.
* Option to invalidate the cache by adding <code>/force</code> at the end of the URL.

@madskristensen Can you create a release in the past? If so, it would be useful to create one for WE2015.0.1.2. For now I let the information for this one directly on the page because I think there is important information (removal of all node.js tools) on this one.