Refactoring

- Break unwieldy stomp_connect() into series of stomp_connection_new(),
  stomp_connection_setcb() and a simpler stomp_connect(). The four callbacks
  can now take their own void * argument.

- Rewrite server message handling in the form of a dispatch table. Simplify
  both stomp_frame_{receive,publish}() functions.

- Remove a lot of the fprintf's and debug, small code reformatting.