Fix crash when unlearning
No need to alias struct.unpack
Use xrange() here
Use with keyword for writing data files
Remove useless unpacking of existing archive on save

Each file in the pack was immediately being overwritten anyway.
Delete unpacked data as soon as it is read
Simplify read_file()
Simplify archive writing
Avoid tempory files when writing data files to archive
Import AtomicFile from https://github.com/sashka/atomicfile

Issue:	20
Use AtomicFile for cfg writing

Issue:	#20
Simplify words/sentences writing
Write words/sentences atomically

Issue:	#20
Update documentation for !purge
Fsync before writing out file