Prefix Exif tag names with 'exif-'
Rename variables in the Image class for clarity

- Rename the 'exif_tags' attribute to 'tags'
- Rename the 'exif_hash' array to 'tag_array'
- Rename the 'non_exif_keys' array to 'non_exif_tags'

Note that the tags attribute returns both Exif and non-Exif metadata.
Upload image metadata