Some improvements of non-local means denoising:

- denoising RGB is now possible, and "colored patches" are then compared

- the main function is now in a pure Python file so that default values
  of kw arguments are visible in the help

- reduced the number of computations of patches bound (but this doesn't
  change much the total speed).

- added an example for the gallery

I also played with functions that could replace the exponential by a
faster and less precise function, but it turns out that most of the time
is spent in additions and multiplications when computing the distance
between two patches.