WebSockets in C Files

Many enhancements to the original concept, the most significant being improved handshaking and ability to signal blocking on send/receive.

I've changed most of the original interfaces.

• added wic_on_buffer_fn callback for allocating a transmit buffer and indicating when socket is not able to buffer
• added buffer type field to wic_on_buffer_fn and wic_on_send_fn to support buffer prioritisation (i.e. so a pong doesn't disappear)
• added wic_on_handshake_failure_fn callback to indicate a failed handshake
• added wic_on_close_transport_fn callback as a dedicated handler for closing the transport
• added wic_on_ping_fn callback
• added wic_on_pong_fn callback
• removed wic_send_pong* interfaces
• changed URL schema to be an enum rather than string
• added mbed port work in progress
• fixed the echo example (was previously closing prematurely)