Specifying the Python' version (2.7x)
Capital letter in mayavi, scipy, numpy etc
Added information to install the Mac OS X dependencies
Added general instructions to install fatiando on Mac OS X

I tried to explain the simplest way (for users).
I wrote **"download Fatiando"** instead of **clone** Fatiando because I
believe this fits better in a "Developers install guide", and the user
do not have to create a github account neither install git.