New definition file and tests for mCustomScrollbar

This is my first time using GitHub for contributing to a project so I'm hoping I've got this right!

Here is a TypeScript definition file and accompanying tests for a jQuery custom scrollbar that I needed to use. I figured seeing as I've used lots of definitions from this project and you don't have this one yet, I would have a go at contributing myself! 

Let me know if I've missed anything. 

Thanks.