Added GIFFormat

The loci.formats GIFReader has been converted to an ome.scifio GIFFormat
and components.

The loci.formats reader has been marked deprecated, and now delegates to
the Format.
Added AbstractMetadata#reset comment

Added commented out code to reset ome.scifio.Fields only. We may want to
implement this in the future.
Format is Prioritized

The ome.scifio.Format interface now extends Prioritized, to facilitate
sorting Formats in a priority order.

The abstract Format implementation always did this by extends
SortablePlugin, but now it is declared properly at the interface level.
Moved isSingleFile and hasCompanionFiles

These methods are now part of the Groupable interface.
Converted MNGFormat

The loci.formats MNGReader has been deprecated and converted to SCIFIO
components: see ome.scifio.formats.MNGFormat.
Added PCXFormat

Converted the loci.formats PCXReader to an ome.scifio PCXFormat and
components. The loci.formats reader has been marked deprecated and now
delegates to the Format.
Converted PGMFormat

The loci.formats PGMReader has been deprecated and converted to SCIFIO
components in ome.scifio.formats.PGMFormat.

The Reader now delegates all functionality to the Format.