Add homebrew formula

[Update] added E-Cell4 homebrew formula to repository.
The formula will automatically resolve the following dependencies:
pkg-config
gsl
boost
hdf5 with --enable-cxx

along with installation of pip and cython, which will require the root password unless installed and will install the following modules:
core(_python)
gillespie(_python)
ode(_python)
reaction_reader

README.rst has been updated accordingly with instructions for installing with homebrew.