Refactor prep.sh to allow independent OMERO and Bio-Formats releases

Test the emptiness of the OMERO_BUILD and BIOFORMATS_BUILD variables to
determine the components to be prepared for release.
For each components, the artifacts checking, the release directory creation
and the artifacts symlinking are conditioned by the existence of the
corresponding build variable.
Add missing echo statements
Merge pull request #21 from sbesson/omero_bioformats_split

Refactor prep.sh to allow independent OMERO and Bio-Formats releases