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.
So -- this is totally cool, but I think we maybe just discovered this doesn't work (at least it didn't for Michael Nagy and I).  building matplotlib from source using conda as dependencies, it doesn't pick up freetype from conda since conda doesn't include pkg-config.  Anyway, if it's working for you @katyhuff, it would be good to know so we can get to the bottom of the cause.