Minor fixes

Refactored code (thanks @xat)
Added ability to start/stop window-resize listener (thanks @xat)
Use % instead of px for positioning, for better scaling
Added shortcuts to plugin methods

Added throttling on window resize updates

Setting image width and height on shell is now optional, but still recommended (thanks @luruke)

Cleaned up variables

This is the first release. If you have any ideas for improvements please let me know!