Added basic code contracts to public api

	1. Added code contract to project compilation
	2. Added not null postcondition to mock factory methods
	3. Added not null postcondition to "Object" properties on
	mock
Optimized code contracts in project file

	1. Added code contracts to release build
	2. Disabled contract analysis for Moq itself
Merge pull request #95 from apeel/master

Basic code contracts for public facing API