Checks if webkit2png is available inside the Screenshot method and now throws an error if it doesn't.

Possible fix for #29
Changed the script to require .start() to be called in order to ensure events have been bound in time.

Changed error event to prevent 'TypeError: Uncaught, unspecified "error" event.'

Issue #29
Merge pull request #32 from firstandthird/feature/graceful-quit

Feature/graceful quit