OS X port of basic spec functionality

This commit adds support for an OS X + homebrew build.

+ added osx and homebrew base environments
+ added OS X Framework Accelerate to provide BLAS
+ updated launcher to avoid bug on OS X
+ maint-commit of petsc4py avoids OS X link failures
+ added Cython as a build-dependency for repository petsc4py
+ generalized OpenBLAS dependency to a BLAS dependency
+ PETSc post-install fixes for Parmetis and Metis library dependencies.