SCIFIO: add convenience methods

Added convenience accessors for remaining SCIFIO services (except two
format-specific services).

Closes #134
SCIFIOImgPlus: attach Metadata

Added API to attach and expose SCIFIO Metadata to SCIFIOImgPluses.

Closes #135
Move populate method to ImageMetadata

Instead of having this method in a static utility method, move it to
ImageMetadata, so it's a little more obvious when to call it.