Changes from 1.4:

hoover.h:
- API NEW: created typedefs for hoovRenderBegin_t,
hoovThreadBegin_t, hoovRayBegin_t, hoovSample_t, 
hoovRayEnd_t, hoovThreadEnd_t, and hoovRenderEnd_t.
This doesn't change the type of any of the callbacks,
but it makes it allows a hoover user to write
callbacks which have specific struct pointers as
arguments, instead of just void*, since the callback
can be cast to one of the types above.

Makefile:
- FILENAME CHANGE: bin/measurend.c --> bin/mrender.c

bin/mrender.c
- callbacks don't have void* args anymore.