Fixed a few exceptions thrown in my environment

Added proper life cycle management for the temporary gzip file. The temporary file could not be deleted as there was no call to the `close` method on the gzip file object. The `with` clause handles the life cycle automatically.

Enlarged the type test on the size variable from 'int' to 'numeric'. An exception was thrown on the assertion statement, as the type was *long* in my environment, and not *int*.

Both previous entries fixed exceptions on my Windows 7 platform, using python-opensubtitles==0.1dev.

Thanks for writing this script!