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.
Ahhhh... I have two sets of each library installed. Glad you caught this. Matplotlib is picking up my macports libpng rather than the conda one! Changing instructions back accordingly.