Initial implementation of aws-hyper (#201)

* Initial implementation of aws-hyper

Although this is expected to evolve, this initial implementation of aws-hyper supports endpoint, signing & response parsing middleware and is sufficient to drive basic AWS services.

A `TestConnection` helper is also provided. This enables end-to-end testing of clients by mocking out the connection with a vector of request/response pairs.

* Update test

* Add more docs, rename default to https

* Add rt tokio feature

* Fix hyper features