Initial version of Bio-Rad .scn reader

See ticket #7703.  All images should be readable, but extra metadata
parsing has yet to be added.
Parse additional metadata from Bio-Rad SCN files
Condense name/value attributes into a single key/value pair
Revert "Condense name/value attributes into a single key/value pair"

This reverts commit bead1d089be61eee0540c68a6360e681f65733ef.
Merge pull request #748 from melissalinkert/biorad-scn

Add support for Bio-Rad .scn files