Optimised buildMap function

[pelrun]

Now uses numpy array operations instead of iterating over elements. Profiling shows runtime on my machine dropped from 7.045s to 0.017s (!!) Should make it much nicer to run on the Pi directly. I've also added an angle parameter (in radians) that changes where the split is placed.

I've just got this running nicely in my own dewarping tool (http://github.com/pelrun/kogetoPi), and there's a rpi camera to kogeto adapter STL in there as well.