Getting rid of devel branch

I'm getting rid of `devel` to make our workflow more similar to other projects like IPython and matplotlib.
* All development should be done in separate branches created from `master`
* Pull requests are from your separate branch to `leouieda/fatiando` `master`
* `master` should always have working code (no incomplete and undocumented functions)
* releases have their separate branches e.g., `v0.1`