Switch meaning of on_open/on_connect

Makes more sense this way.
endpoint: Support sending reason-less close

Support passing in 'NULL' for the close reason in endpoint_close
Makefile: Add heelhook_static and heelhook_shared

Add the ability to build heelhook as a static library (heelhook.a) or as
a shared object (heelhook.so.1.0)
config.h: Minor cleanup
server: Add should_stop callback

Add the ability for the user of the server module to specify a
should_stop callback. It will be called periodically, and if it returns
true, the server will do the same thing as server_stop.

This is useful if you know you want to stop the server but don't have
access to the server object itself to call server_stop (that is, a
signal handler).
Add heelhook python C extension

Yay, now you can make a server in python.
darray.h: ssize_t technically needs unistd.h
Update README with python module