Added ability to clear cache for a repository

Fixes #68

Still need to find a way to do a global clear cache, which will be
located at Cache.ClearAll() most likely
ClearAll for CompoundKey CachingStrategy
Removed the Dynamic LINQ helper and replaced with Expressions

This is a cleaner more performant approach.  Had to make extension
methods for OrderByProperty and OrderByDescendingProperty that take a
string for the property name
Changed MongoDb GetQuery to use FindOneById