Correct a python vs python3 inconsistency in readme.
Adjust file.file_mtime() for nanosecond accuracy.

This is needed for correct comparison vs the timestamps TMSU stores in the database.
I would like to return a datetime object, but datetime doesn't currently support
nanosecond resolution. Hence, a string. Hopefully SQLite will store it as a 64bit integer
rather than a string.

strftime() also doesn't have format codes for nanoseconds, hence my
string concatenation hack here.
file_mtime: more adjustments to format.

Should exactly match TMSU now. The incoming 'tmsu.repair check' CLI
was used to test this -- it has a 'badmtimes' diagnostic,
which found 24000+ files in my DB which had a subtly incorrect date.
repair: new: changed_paths(), with CLI. (dry-run only) update_file_metadata CLI also added.

changed_paths() detects various error/warning conditions in the `file` table,
categorizes and records them in a {category:[filenames]} dictionary.
The CLI, without --json, simply reports stats on these.
With --json, it also outputs a JSON file to stdout.

Categories of error/warning condition include:

* badmtime (mtimes off by <1s. Should be fixed by updating mtimes
            to match on-disk mtimes for the paths)
* broken (broken symlink. Usually needs manual fixing or removal)
* modified (mtimes differ by >1s, path is probably modified and needs re-fingerprinting)
* deleted (path no longer exists.
           taggings should be merged with an existing duplicate if possible,
           then the path should be removed from file_tag and file tables)
* type (path pointed to a directory, but now points to a file, or vice versa)