Texture synthesis implementation of Efros and Leung's algorithm

NumPy implementation of Efros and Leung's algorithm. Currently, refraining from Cython implementation since profiling results show that the majority of the time is spent on ``np.einsum``. 

The results are great for generating recurring textures in the image and in this regards is better than FMM as well, although a little slower. Check the example in the docstring, texture synthesis performed on ``skimage.data.checkerboard`` Algorithm is able to perfectly reconstruct the texture maintaining the crisp edges.

Docstring, unit tests, general example and an interactive demo added.

@emmanuelle