New LCurve class to estimate regularization parameters

The new class was added to `fatiando.inversion.regularization`. It behaves like any other solver but under the hood it runs the L-curve and finds the corner value when `fit()` is called. 

The L-curve is built using user-specified values of the regularization parameter. The corner value is found using the "triangle method" of [Castellanos et al (2002)](http://www.sciencedirect.com/science/article/pii/S0168927401001799).

[This gist](http://nbviewer.ipython.org/gist/leouieda/9099363) was the first prototype developed.

Notes:

* So far it's not behaving well for total variation regularization. **Update**: got this working on the SrTomo recipe. It was just a matter of adjusting the search range of regularization parameters. Very large values were giving errors.

TODO:

- [x] Use it in recipes
- [x] Fill docstrings
- [x] Compute the curve in parallel when possible
- [x] Changelog entry