Ability to run functional tests on all OS

For now, the functional tests were ran with:

• nuxeo-distribution/nuxeo-distribution-dm/ftest/selenium/run.sh
• nuxeo-distribution/nuxeo-distribution-dm/ftest/funkload/Makefile

First step:

• develop an os-independent solution for running Selenium tests with Firefox under Linux, Mac, Windows.

Next steps (outside this issue perimeter):

• run tests with IE under Windows.
• adapt Funkload scripts
• remove deprecated run.sh