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 don't think this'll cut it. Either we move the whole thing out, or we skip this altogether. It's way too dirty to have a reference to the impl. in an interface IMO.