Doc reorganization (rebased onto develop)



This is the same as gh-315 but rebased onto develop.

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Following the merge of #296, this PR reorganizes the documentation as follows:
- creation of an `omero/` folder where all the OMERO documentation is moved
- creation of a `common/` folder for shared items (images, configuration, themes)
- extraction of shared configuration settings into `common/conf.py`
- creation of a main `Makefile` to build the HTML & PDF versions of both documentations.

To test this PR, 
- check the Travis build passes (should test the HTML build of both docs)
- check the Jenkins build is still green (after configuration modification)
- check the HTML & PDF output for both documentations are unchanged