Install instructions for OSX

@leouieda  I wrote the install guide for Mac OS (one possibility), but I'd like to suggest that all the install guide be restructured. To me, is a little bit complicated talk about the `dependencies` and its particularities for one OS, and after go back to explain how to install Fatiando in that OS. Even because in the Mac OS X case, I suggested the use of another `dependence`. 

Solves #23 

I imagined something like this. What you think?

Installing Fatiando
=============

General information
----------------------------

* Python version
* Dependencies (list of dependencies of fatiando)

1. Linux installation
-----------------------------

1.1 Dependencies

1.2 Easy way / Using Python-IDE or binary installers

1.3 Hard way / By hand or something like this  (building...)

1.4 How to test?


2. Windows installation
---------------------------------

2.1 Dependencies

2.2 Easy way / Using Python-IDE or binary installers

2.3 Hard way / By hand or something like this (building...)

2.4 How to test?


3. Mac OS X installation
------------------------------------

3.1 Dependencies

3.2 Easy way / Using Python-IDE or binary installers

3.3 Hard way / By hand or something like this  (building...)

3.4 How to test?

4. Developer Installation
-----------------------------------

2.1 Dependencies

    exemple: git and github account

2.2 Instructions

2.4 How to test?

2.x Proceedings or something like this