Texture synthesis implementation of Efros and Leung's algorithm

Implementation of Criminisi et al. algorithm. Great results for both textural synthesis as well as structural case. `plot_inpaint_criminisi.py` shows the real effectiveness of the algorithm. 

Profiling results: 2500 pixels in 10.4 sec.

Things to be done:
1. Cythonise `_inpaint_criminisi` function

@emmanuelle @sciunto @stefanv review please!