Add a helper class for managing OMERO sessions

This class encapsulates session-specific state, so that the
OMEROFormat.Metadata does not have to do so.

I.e., it is wrong for the Metadata to have sensitive transient state.
Rather, it is the job of the Parser to initialize a session based on the
credentials, then populate the metadata with format-specific information
obtained from the session at parse time. Then the Metadata's
populateImagMetadata can translate that format-specific information into
ImageMetadata without needing access to the OMERO session.
OMEROFormat: set default port to 4664
OMEROFormat: add missing accessors
OMEROFormat: add more format-specific fields

The idea is for these fields to be populated at parse time, so that the
Metadata does not need to keep an OMERO session as internal state.
OMEROFormat: get metadata from OMERO while parsing

The previous implementation created an OMERO session and parsed the
metadata within the Metadata itself, which is a violation of SCIFIO
principles. Now the metadata parsing actually happens during parsing,
and the OMERO session is no longer stored in the Metadata itself.