Add link to OMERO Homebrew installation script to the doc page
Explain system-wide Python vs Homebrew Python installations

Note this commit does not make a final choice on a recommended strategy for
Python. The goal here is rather to present both alternatives and point at the
differences in the corresponding sections of the documentation page.
Expose the installation formula with Ice 3.4
Fix environment variables example block

- Remove deprecated BREW_DIR variable (since we are documenting the default /usr/local)
- Remove $ICE_HOME/python from DYLD_LIBRARY_PATH
Add links to the Homebrew and Python wiki page in Python section

Also add warning about `brew install` and virtual environments
Fix incomplete sentence and multiple spaces reported by Blazej
Remove all mentions of BREW_DIR
Replace Note the by Note that the
Add sentence about virutal environment and decrease warning to note

Also add a link to the virtualenv website so that sysadmins know what virtual
environments are about.
Clarify the Python dependencies installation site-packages paragraph
Merge pull request #445 from sbesson/rebased/develop/11361_homebrew_python

Homebrew and Python (rebased onto develop)