use the new chrome.commands API rather than external key logger

This branch refactors the entire codebase to leverage the newly released chrome.commands API. I have not tested all music providers, so some help would be appreciated there. (Especially vkontakte & yandex as those were modified significantly)

The chrome.commands API is still new, but it is fully available in latest public Chrome builds.

Please note when testing, that the user *must* access the `Keyboard shortcuts` section of [chrome://extensions](chrome://extensions/) after installation in order to "approve" the Media Keys as "Global" shortcuts.  The keys will not be captured until this step is taken.