Use Hyper instead of Rust-HTTP

Fixes #172 by using Hyper.
Fixes #217 by changing the ContentType modifier to take a full Mime type.

Regressions:
    - We no longer set the content type automatically when using Path and
      the Body modifier. Before this can happen, rust-http-content-type needs
      to be updated or replaced.
    - We have new (safe) unwraps in the codebase and examples.

On the other hand.. no more rust-http!