Add basic unique indices

Has some limitations:
* Raises an new error, `Nonunique`, but only when index is re/built.
* Only works with file backends.
* Works every time the method `.add_hashed_value` is called. 
* Tests are pretty basic, and don't fully utilize py.test capabilities.

This could be the starting point for a discussion on what this should look like, or could just be committed, if you think it is OK.

Sorry for whitespace diffs.