Enable Refit on iOS via code generation

*This is on-hold until Xamarin fixes the iOS build system on OS X so that we can add pre-build commands*

This PR :fire:s Castle.Core's dynamic proxies and replaces them with code generation via analysing the existing source files with Roslyn and running a pre-build task which generates a new "RefitStubs.cs" code file. 

### TODO:

- [x] Find a more reliable way for InterfaceStubGenerator to find the project directory (probably by doing hacky things)