DefaultOMEROService: make service optional

That way, if the DefaultOMEROService cannot be instantiated for whatever
reason (such as OMERO server JARs not being on the classpath), then the
ImageJ context does not come crashing down.

This lets us launch ImageJ the normal way straight out of
$OMERO_HOME/lib/ImageJ.app -- although OMERO-specific functionality such
as File > Import > OMERO will not work in that case...
DefaultOMEROService: stop using deprecated methods

The conversion routines of ClassUtils moved to their own class:
ConversionUtils. So let's use that instead now.