Switched server engine to using zloop

- added handle_socket () hook for application sockets
Engine API methods all start with engine_

- Meant to make methods more consistent with the CLASS standard.
Exit properly on Ctrl-C

- has to signal to zloop that a recv failed
- has to only do terminate handshake when destroyed via API
  rather than reacting to Ctrl-C
- also check that bind commands succeed (assert if invalid)
Fixed 'echo' handling in config files

- was incorrectly looking for echo at wrong level
Expand headers before BNF

- also cosmetic indentation fixes
Merge pull request #44 from hintjens/master

Improvements to engine generator