Get rid of fatiando.io module

I will remove the `fatiando.io` module. Functions that download datasets will go into a `fatiando.datasets` module/package inspired by the [scikit-learn datasets package](https://github.com/scikit-learn/scikit-learn/tree/master/sklearn/datasets). The IO functions for Surfer grids will go in `fatiando.gridder`. `fromimage` will go into `fatiando.utils` for lack of a better place.

Solves #66 

PR #37 should be modified to conform.