Added ability of engineer settings and added to D2K

I added ability of engineer settings works like Tech Levels. #5986 
![image](https://cloud.githubusercontent.com/assets/7933210/3685025/d5e0af7e-1304-11e4-9264-9bff90e6b6be.png)
But it brokes crates at only D2K.(I don't know it's couse)
http://www.mediafire.com/view/flb7dm77lo0fp2y/exception2.log
Aka in OpenRA.Mods.RA.LintBuildablePrerequisites and add
var engineerPrereqs = map.Rules.Actors.SelectMany(a => a.Value.Traits
				.WithInterface<EngineerPrerequisite>())
				.Select(p => p.Prerequisite);
and modify
var providedPrereqs = buildingPrereqs.Concat(customPrereqs).Concat(techPrereqs);
to
var providedPrereqs = buildingPrereqs.Concat(customPrereqs).Concat(techPrereqs).Concat(engineerPrereqs);