Add custom icon support.

• Create folder are artwork
• Add textures to the icons lua file
• Add section to the dev guide
• Add examples

Recommend 256 x 256 targa created in GIMP. Photoshop can be problematic sometimes. Add "how to" to the doc ticket #4

Doc added here: https://github.com/lorikrell/hn-wconquest/wiki/Add-Icons-and-Images#add-a-custom-image-icon

Ideas for designing icons:
https://www.pinterest.com/pin/611574824379755503/
https://www.designcrowd.com/design/9174901
https://www.pinterest.com/pin/261631059586121015/
https://eldamaranquendi.tumblr.com/post/186678077782/lord-of-the-rings/amp

Added custom icons for the events. Expanded events list. Adding this info to the wiki.

Complete!