README: Remove double spaces
Update Homebrew installation page to expose Ice 3.4 as the default version

- Describe Ice 3.4 as the default version (e.e. as installed using ``brew in
- Explicit alternate ways to build OMERO with Ice 3.3
- Fix environment variable to reflect the default choice
- Remove deprecated comment in the Ice 3.3 troubleshooting section
Fix envvar markup
merge develop -D org -I include -E exclude: PR 369 (Cpp: Correct content and markup)
merge develop -D org -I include -E exclude: PR 437 (Generate PDF using XeLaTeX)
merge develop -D org -I include -E exclude: PR 448 (Managed repository)
merge develop -D org -I include -E exclude: PR 450 (omero: Python 2.6 is the minimum supported version)
merge develop -D org -I include -E exclude: PR 457 (Update Homebrew installation page to expose Ice 3.4 as the default version (rebased onto develop))