add support for generic resource overlays

This is a first attempt at adding generic resource overlays.

For now, this means supporting:
* [Kethane][Kethane] resources:
  * Ore
  * Kethane

* [Open Resource System][ORS] resources, including:
  * Uranium
  * Thorium
  * Alumina
  * Water

An image gallery to see examples of this can be found [here](http://imgur.com/a/gpU1i).

[Kethane]: http://forum.kerbalspaceprogram.com/threads/23979-Kethane-Pack-0-8-5-Find-it-mine-it-burn-it!-0-23-5-%28ARM%29-compatibility-update
[ORS]: http://forum.kerbalspaceprogram.com/threads/64595-Open-Resource-System-%28ORS%29-Mod-Resource-API-version-1-1-0