ImageMaps

Huge shoutout to BaronyCraft for contributing the new features and fixes!

* Prevent users from escaping the /images folder
* allow reloading images from console
* cancel pending placement by right clicking air (requires item in hand)
* added option to download image
  * /imagemap <fileName> download <URL>
  * requires imagemap.download permission
* added option to scale images on placement
  * /imagemap <fileName> scale <blocksX> <blocksY>
  * scales to fit into the given contraints, but keeps aspect ratio
* added info command that displayes images sizes
  * /imagemap <fileName> info
* now display the size requirements when placing an image