total variation denoising

[Emmanuelle Gouillart]

> The code looks great.  Two minor things: a) let's move the tests into
> tests/test_tv_denoise.py b) Document the input dtype--currently data
> needs to be in floating point format--should it be in [0,1]?  This is
> not quite clear.

Done; please tell me if it's ok. The input data can have any numerical
dtype, actually, it doesn't have to be in [0, 1].
Cheers,
Emmanuelle