OSX install

This makes some slight instruction changes for the README in the OSX README and adds a little updating in the INSTALL file. This is only a solution to #3209 for the OSX platform. Someone should still check and change the windows instructions, for example, as discussed in issue #2999.
I would concatenate those two sentences.
"The Continuum.io Python distribution Anaconda and the Enthought distribution canopy are both excellent choices that just work out of the box". This way we are not putting one above the other (politics…).