Now finds the "add to cart" button properly

A bit more general cleanup, as well.

Better usability

Messaging tweaks.

Better handling of missing/invalid API keys.

Added back Chrome devices, listed separately, with a warning

0.0.8

Corrected matching pages and bumped version.

Now properly matches all device details pages.

Oops.

Bettery error messaging

Now *only* functions on device details pages.

Version bump.

Better device listing.

0.0.5

Better device listing.

Changed method of checking if device is available

Now it checks if we found an enabled "add to cart" button, rather than clicking blindly and hoping it worked, checking if we arrived at our cart after our refresh interval.

Because of that, it now calls PushBullet quicker than it did before.

Now actually works!

Fixed a bug that prevented pushes from being sent when the device was available for sale.

Improved usability by removing the ability for users to shoot themselves in the foot. Also added messaging for two conditions where no PushBullet devices are available to push to.