speed up

Don't compute grad wrt output for first layer of nn
incomplete C code for lua nn package

I built this because I wanted to compare timings in the C code to
timings in the optimized lua code. The C code would need to use
gsl and glib (or similar) so that the code would take advantage
of any available BLAS implementation.
condense printout
count number of times each method is called
new: maintain table of accumulators
specify improved API (not implemented)
validate arg
new: time laps (=code sections)
clean API to access lap timings
remove lap timing capability (now in LapTimer)
improve source code layout
documentation
reset timer at end of lap
start lua, not luajit
rename
also return number of times each method called
instrument for timing
new: select k nearest neighbors with equal weight
remove debugging code
correctly handle when function name is missing