Remove OMERO documentation set images folder and add to .gitignore
Add bash script to rsync remote images on necromancer

This script requires rsync to be installed as well as necromancer to be properly set up under .ssh/config. The BRANCH variable allows to choose the documentation images set to be downloade - default to dev_4_4_staging.
Fix BRANCH variable default value
Add sync_images to Travis build
merge dev_4_4 -D org -I include -E exclude: PR 368 (Cpp: Correct content and markup)
merge dev_4_4 -D org -I include -E exclude: PR 407 (Move OMERO images out of the Git repository)