Makes the crate's API way more understandable (#96)

* Move common to crate root

* Move transport traits to own module

* Hide transport::client and transport::server

* Move ws and http to transport

* Typo fix

* Remove the shortcuts for raw client and servers

* Rename core to raw

* Remove commented-out code

* Fix docs of raw module

* Rustfmt

* Force inline

* Rustfmt