Add NewRawRequest

[milesbxf]

One common pitfall of using typhon.NewRequest for standard HTTP usage is that it's easy to try and use it like a regular http.NewRequest and pass in a raw body, expecting that to work - but then Typhon encodes the body as JSON. Now of course you could pass in an io.ReadCloser/io.Reader to avoid this, but you'd only know this from digging deep into the Typhon code, not from the docstrings!