Enable in derived modes of `page-break-lines-modes'

For example if `page-break-lines-modes' contains `prog-mode',
`global-page-break-lines-mode' will apply to `python-mode', `c-mode',
etc.

Tested on Emacs 24.2.
Merge pull request #2 from drothlis/derived-modes

Enable in derived modes of `page-break-lines-modes'