Asynchronous HTTP client and Websockets support

Websocket support (#46)

* http headers are case insensitive

* update dep: libuv -> v1.38.0

* refactor http_src_t

* implement websocket
* refactor HTTP req
* refactor TLS link

* portable endian ops

* init tls state

* send pong when receiving ping

* update req_body_len so it matches the header

* check content-length header

* try fixing TLS client cert test

* memory cleanup

* graceful close

* doc

* fix copyrights

* cleanup

* support arbitrary number of response headers