Limit ColorTable instantiation

Instead of always wrapping color tables in Bio-Formats in new ColorTable
instances, we will try to cache generated ColorTable objects via weak
mappings, keyed on the Bio-Formats table.

Closes #8
Merge pull request #9 from scifio/color-table-cache

Limits repeated wrapping of color tables from Bio-Formats by creating a weak map to cache `ColorTable` instances.