Encode the world

This release breaks:

• Any use of the encode-body and coerce-response interceptors in the clj-http, httpkit or cljs-http namespaces. These interceptors have been unified under martian.interceptors, see #23

This release adds support for:

• Supporting custom encoding in addition to the built-in support for transit+msgpack, transit+json, edn and json #23

This release fixes:

• Identical requests sent while the previous request is still pending are deduplicated in martian-re-frame #48

This release improves:

• Documentation on per-route interceptors #49
• Documentation on providing body parameters #50