Add LensID in possible tag values

LensID is a tag that provide information to identify lens type.
For example, "241" means "Canon EF 50mm F1.2L USM".
It's not always a numerical identifier, it can be a string that give informations about the lens type.
Be careful, lens id is not always specified in exif metadata.
Merge pull request #10 from mjeanroy/master

Add LensID in possible tag values