Throw a better exception when the user didn't add the MVC services

Issue #347 - Added a check in UseMvc() to verify if AddMvc() was called before.
Added functional tests for the same.
Agree. This is a good idea. Will implement this in the next iteration.

Would like to point out one corner case though - If the user decides to add all the services one by one manually (for some reason) then he might miss this service (which we are going to specifically use to validate AddMvc()).