Class to estimate magnetization diretion of dipoles

The DipoleMagDir class estimates the magnetization vector
of dipolar bodies with known center. The estimate is
obtained by from total field anomaly.

TODO:

- [x] add at least 1 cookbook recipe
- [x] write docstring for new module magdir
- [x] add magdir entry on `gravmag/__init__.py`
- [x] check if all tests pass (`nosetests --with-doctest -v fatiando test`)
- [x] place a gravmag.magdir.rst file in the doc/api folder (copy something like this: https://raw.github.com/leouieda/fatiando/master/doc/api/gravmag.euler.rst)
- [x] add `gravmag.magdir.rst` to the list in this file: https://raw.github.com/leouieda/fatiando/master/doc/api/gravmag.rst
- [x] add an entry to the changelog: https://raw.github.com/leouieda/fatiando/master/doc/changelog.rst

Since this is the first new feature, you'll need to update some things on `doc/changelog.rst`:

Add a new section for version 0.3:

    Version 0.3
    ---------------
    
    **Release date**: 

    **Changes**:


Then after that you can make an entry describing your change.