Introduce rake and Centroid

This refactors the repository to use `rake` to drive all automation. Centroid and `config.json` keep the Simpler specific information out of the Rakefile.

This also removes the NuGet packages (except for NuGet.CommandLine) from the repo. Packages used by the solution will be automatically downloaded by Visual Studio (or the latest version of Xamarin Studio). The other option is to run `rake install`. `rake install` is required to run other rake tasks.

The rake tasks are shown below.
```
$ rake -T
rake build:debug    # Build for debugging
rake build:release  # Build for release
rake clean:all      # Clean all output
rake clean:release  # Clean release output
rake clean:test     # Clean test output
rake default        # Run tests
rake install        # Install NuGet dependencies
rake release        # Pack and push NuGet package
rake release:pack   # Pack NuGet package
rake release:push   # Push NuGet package
rake test           # Run tests
```

Tasks
- [x] ~~Document details of how everything works in a CONTRIBUTING.md file.~~ #22 was created instead.
- [x] ~~`bump` tasks only work on Windows.~~ These tasks were deleted for now.


This will close #18.