Added inplace_increment function 

I've added an inplace_increment function which allows you to increment an array inplace using advanced indexing. The key feature is that repeated indexes will repeatedly increment the array whereas 

a[idx] +=1 

will not do this. More discussion [here](http://www.mail-archive.com/numpy-discussion@scipy.org/msg37949.html) and [here](http://comments.gmane.org/gmane.comp.python.numeric.general/50291).

I've included, documentation, a couple of tests and supported multiple datatypes.

I'm new to doing Python functions in C so a good once over of the C code is a good idea. I'm also new to submitting code for numpy, so I've probably made some mistakes somewhere.