Added the Mojio Library and created a simple login screen.

Of note is that during compilation, it warns that the methods used are depreciated and that we should use the Async versions. However, I can't find any mention of these new API's. 

I have also added .userprefs into the .gitignore.

Anyways, I pushed so we have a new base to work on.

Files changed aside from adding the Mojio Library: ".gitignore", "MainActivity.cs", "Resources/layout/Main.axml"