v2.0.0

This version is a complete rewrite of the library; apologies that v1.x was so short-lived.

The primary motivation behind the rewrite was not-wanting to force consumers to inherit from a base class just for the purpose of overriding one method. This rewrite uses interfaces for specification expressions and functions rather than base classes. It also moves all of the 'utility' functionality into extension methods. Whilst extension methods are static and cannot be substituted, these extension methods are very small, have focused well-understood functionality and should have no reason to ever change.