update gemfile for `photor search` sqlite3 db
make `photor index` and `photor search` use CWD by default
rename out of confusing Photor::File constant
tweak file unique name pattern

in practice, i still want the full date in file names. this helps when an import
or a search result spans multiple days.

the md5 portion doesn't need to be full length. 24 chars is more than enough to
deter fuskers, and is also more than enough to prevent collisions in file data
(especially paired with the full datetime).
fix rename to not rewrite valid names
test framework