Create metadata class

[SamWhited]

Create a class to handle metadata.

Basically just a named tuple or dict with specific names for the various metadata fields. This will get quite large. A named tuple is probably the best way to go, but it also seems like it would be a pain to maintain.

It may also do some basic verification / normalization of data, though some of that may also belong in the individual raw format classes.

Downstream issue: photoshell/photoshell#82

[SamWhited]

Blocking on #9