Remove Git configuration sections from the developer documentation

Assuming this section only deals with the OMERO installation from source,
the Git configuration for contributing is irrelevant here and is also a
duplicate of the Contributing documentation.
Add placeholder for OMERO build dependencies
Add link to contributing documentation
Re-organize paragraphs
Typo fixes
Add source code downloads paragraph and add link to contributing for Git
Fix the environment variable name
Add note about current caveat of the OMERO source code zip
Remove references to ome.git
Change Installing to Building
Add example dev requirements for CentOS 6 and Ubuntu 14.04
Remove brew deps in favor of brew install --only-dependencies

As reported by @kennethgillen, the current command to install OMERO
dependencies only does not work properly with dependency options. This
commit uses the Homebrew option for installing formulas with dependencies.
Adding 5.0.4 to OMERO version history
Moving walkthros up a level in TOC
Merge pull request #957 from sbesson/rebased/dev_5_0/dev_build

OMERO developer setup  (rebased onto dev_5_0)
Adding OMERO.matlab fix to history
Merge pull request #965 from hflynn/rebased/dev_5_0/walkthro-links

Moving walkthros up a level in TOC (rebased onto dev_5_0)
Merge pull request #960 from sbesson/homebrew_deps

Remove brew deps in favor of brew install --only-dependencies
Merge pull request #964 from hflynn/5-0-4-version

Adding 5.0.4 to OMERO version history