Remove reference to 4.3.4 server
Move OMERO installation section before requirements

With the recent optional dependencies added to the OMERO formula, `brew install omero` already installs all the dependencies required to build the
server.
Update OMERO & Python dependencies installation using Homebrew

- Add `brew tap ome/alt` command.
- Move PostgreSQL and Python dependencies installation after OMERO
installation
- Add command to install OMERO dependencies only using Homebrew
Merge OS X and Xcode sections and remove OS X <=10.5
Add section about 32-bit gfortran and fix various text formatting
Minor documentation fixes suggested by Mark
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 262 (Training develop)
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 265 (Review the Homebrew installation page (rebased onto develop))