Review the Homebrew installation page

This PR updated the Homebrew installation page to reflect the changes brought:
- to the Homebrew formula including the new style dependencies (ome/homebrew-alt#20) and the 4.4.6 bump (ome/homebrew-alt#21) 
- to the Python dependencies installation script (openmicroscopy/openmicroscopy#764)

Summary of changes:
- modify the installation order as 1- Homebrew, 2- OMERO, 3- Postgresql, 4- Python deps
- merge & update the OSX & Xcode requirements sections (10.5 no longer tested)
- fix various text formatting markup
- add a section about failing ``brew install gfortran`` under 32-bit Snow Leopard