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
Merge pull request #954 from sbesson/dev_build

OMERO developer setup