Moved View/Negotiate to INancyModule

Superseeds PR #1472

Add's missing XML Comment and moves View/Negotiate to the `INancyModule`

This allows the module to be mocked for use in 3rd party app testing.

i.e in SimpleAuthentication library we have an `IAuthenticationProviderCallback` class which has the module passed into, however this cannot be tested without attempting to mock the entire NancyModule. 
I'm actually not happy about this, I did consider pulling it out, but because there's plans to do a refactor of all this stuff I decided not to. Any refactoring of this is liable to create a breaking change. 

The pulling to the interface solves the immediate problem of testability. I believe we can worry about this when we do refactor and don't mind any possible breaking changes.