Fixed ImageMetadata population

When populating ImageMetadata using an OMEXMLMetadata, was accidentally
counting size Z as both Z and T axis lengths. Fixed.
Changed default calibration values

ImgOpener should now populate default calibration values of 1.0 instead
of NaN, to allow computations that assume calibration values to be
performed.
Fixed ImgSaver workflow

Updated signatures to prevent infinite loop/NPE

Also force translation to SCIFIO Metadata, to trigger format-specific
translation processees.
Moved ReadImage to scifio unit tests

Updated for ImgOpener modified API. This allows the IO component to be
completely removed from imglib.
Added ImgOpener flags.

Should really consolidate these signatures to a single key,value list...
Added getKey method

Guarantee keys are kept in memory and consistent keys are being used in
ehcache.
Populate ImageMetadata before parsing IJ comment

The image metadata must be populated before parsing IJ comments.

Added a toggle to TIFFFormat to determine if ImageMetadata was
already populated, to prevent overwriting by MinimalTIFFFormat.
Javadoc for ome.scifio.io.img.*

Added comments, licenses and package-info.java for the ome.scifio.io.img
package and sub-packages.
Added status message when opening CellImgs

Added a general status message to ImgOpener when a CellImg is created.
Status events should not be displayed each time an individual cell is
loaded, as the progress bar would be arbitrary and this can lead to
blocking behavior.