#60 plugins cannot be removed on Windows in 2.6pre1

[Slava Pestov]

To fix this, will need to revert to old 'backup directory' code, and remove
this backup directory on exit. Alternatively, could add a close() method to JARClassLoader.

[Slava Pestov]

Fixed, by adding a close() method to JARClassLoader, in 2.6pre2.