Correct behavior of FileChangedChecker garuanteed.

The FileChangedChecker class has been changed. A new class
`FileChangedChecker.Worker` has been introduced to dispose itself from
the list of workers that have been stopped (to make sure the workers can
terminate peacefully).
extended c4dtools.prepare()

`c4dtools.prepare()` can now also parse description symbols. The
argument `parse_descriptions` must be set to True for this. Note that
strings can not be loaded from description symbols.

Further Changes:
- `c4dtools.prepare()`, argument `cached_res` has been renamed to
`cached_symbols`.
- `c4dtools.resource.load()` now turns unicode-strings returned from
`json.load` into string objects.