Use os.fs{decode, encode} methods for plain SHA512SUM importing
Allow filenames that can't be decoded in the system locale

In Python 3.4, these are only problematic when passed to print(). Replace them
with U+FFFD REPLACEMENT CHARACTER when dumping file lists.

We can pass the surrogate escape code points to the JSON module and they'll be
serialized without any problems as per http://bugs.python.org/issue17906.
Add 'split' action and associated documentation

This reads an existing hash database, filters entries that are not contained
in a specified subdirectory, and then writes the filtered database to that
subdirectory.