Abstracts fact fetching for Github sources

[brendo]

This builds on #58, by introducing a GithubSourceFacts class which will use the Github API to retrieve the information. This is largely a refactor of existing code, with the additional of implementing sessionStorage caching.

[brendo]

@squidfunk Sorry, this seems to have a couple more files than what I was expecting. What flags are you using with gulp prior to commit?

I noticed release fetching in your latest commit, and the start of abstraction. If you're happy with the approach in this PR I could roll that in as well. Up to you!

[squidfunk]

You should run the build before the commit. Only the scripts in package.json should be used:

npm run build

I will rework the documentation to reflect that and add a CONTRIBUTING.md However, as everything's in progress, it's doesn't really matter.

I'm sorry for all the mess in application.js, I'm still in prototyping-mode. Everything will be cleaned up or the release. Thanks for abstracting this stuff.

[squidfunk]

BTW: good idea on using the session storage for the repository facts - was on my internal list. Did I mention it somewhere here or was that your own idea?

[brendo]

That was your idea, I stumbled across the to-do while abstracting the logic. Thanks for the merge!

[squidfunk]

Awesome! Thanks for going ahead and doing it.