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.
yeah, I think you're right. This bit was already in there and I thought about changing it, but I thought maybe (due to the discussion of linux later) there was something subtle about matplotlib on debian systems or something. Not the case?