Explicitly disallow Python 3 and Ice 3.5 in stable docs

With the new release of Ice 3.5 we need to make it immediately
clear that Ice 3.5 is not (yet) supported. Similarly, Python 3
is not an option, whether with or without Django involved.

See forum thread:

 * https://www.openmicroscopy.org/community/viewtopic.php?f=5&t=4788&p=9654#p9654
Merge pull request #322 from joshmoore/ice35-py3-fix

Explicitly disallow Python 3 and Ice 3.5 in stable docs
Move OMERO documentation to omero/ subfolder
Fix compilation by pointing at the themes/ directory
Remove formats from excluded patterns in OMERO documentation
Move shared configuration settings to common configuration file

- Common configuration moved to common/conf.py
- Move themes to common/
Add central Makefile to build OMERO & Formats documentations at once

Also allows Travis script command to remain unchanged