Added AMD Module Support

Hello, thanks for this nice library. I made a small change to the library for my purposes and thought that others may benefit from the change.

I've cleaned up the module handling so that jscache is supported by AMD module loaders. This benefits people using libraries such as requirejs to build their web applications.

The existing loader support remains the same. The Node/CommonJS style module continues to work, as well as the simple ```<script src="jscache.js">``` approach.

The one breaking change is that the CachePriority enum needed to be brought in to the module. This means that ```CachePriority``` is now ```Cache.Priority```. For example, ```Cache.Priority.HIGH```.