Try to use cpufreq-set with user authorization

Hi,

First of all, thanks for this extension :).

This patch tries to use `gksudo` or `sudo -A` to allow user to switch governor when there is no `cpufreq-selector` available, e.g., on Ubuntu 13.04.

There is also a part using `pkexec`, but it does not work currently, because of a problem found elsewhere (i've included TODO comment with explanation). That's why it's commented for now.

I've changed `cpuFreqSelectorPath` to `cpuFreqSelectorCommand`, to make it more flexible and usable for different commandlines.

Hope it helps :).

Regards,
ahwayakchih