Add proximal opeartor for the 2D total variation penalty

[fabianp]

Also includes an example of using this proximal operator for denoising. The output of this example is

Other changes:

• Added tolerance parameter for FISTA

[mblondel]

Nice!

I have recently worked on a more efficient implementation of FISTA:
https://gist.github.com/mblondel/5105786d740693a6996bcb8e482c7083

The main advantage is that it is possible to compute the projection and regularization term in the same function call, which allows to share some expensive computations in some cases (e.g. nuclear norm). I also support a tolerance constant.

[mblondel]

@fabianp I am not very familiar with the TV proximity operator. Perhaps you could ask someone else who is to give a quick review?