Minor docstring adjustments and error checking
New: untag_files, delete_file_tag, delete_file_taggings.

Which respectively

* untag files
* delete all instances of a tag from the `file_tag` table
* delete all taggings of a file from the `file_tag` table
Dead simple API for reading the settings table
Fingerprinting api: fingerprint(), get_fingerprint_algorithm()

Tested successfully but far from exhaustively against my main TMSU database.
file_mtime() : new, returns suitably-formatted mtime of file for storing in db.
repair: new submodule

Currently the following functions are available:

 unknown_paths, invalidated_paths, duplicated_paths

 rank_duplicates() is also available,
 to sort the groups generated by duplicated_paths() by
 'categorizability' and 'filename descriptiveness'.

Use of other functions should be avoided.

Also allow core.splitpath to be imported.