Commit to add "Coverage" option to procedural terrain generator

This commit adds a "Coverage" option to the procedural terrain
generator. Allows mixing of textures. See
http://www.garagegames.com/community/blogs/view/22371 for more info.
This version has many improvements - coverage is now entered as a
percentage, accepts decimals, and doesn't have any bugs as far as I
know.
Merge remote-tracking branch 'JackDavidson/master' into procedural-terrain-coverage
Fix random number generation.
Merge pull request #687 from eightyeight/procedural-terrain-coverage

Add coverage option to procedural terrain generator