Problem Exporting WebApp because of extra dll's

Hello Everyone, I'm trying to make a game that use a third party library that i included in the Plugin folder(a .DLL). My game works fine in desktop but no way to make it build for webapp purposes. This is the error message I got:

ArgumentException: The Assembly System.Drawing is referenced by FortuneVoronoi ('Assets/Plugins/FortuneVoronoi.dll'). But the dll is not allowed to be included or could not be found. UnityEditor.AssemblyHelper.AddReferencedAssembliesRecurse (System.String assemblyPath, System.Collections.Generic.List`1 alreadyFoundAssemblies, System.String[] allAssemblyPaths, System.String[] foldersToSearch, System.Collections.Generic.Dictionary`2 cache, BuildTarget target) (at C:/buildslave/unity/build/Editor/Mono/AssemblyHelper.cs:154) UnityEditor.AssemblyHelper.FindAssembliesReferencedBy (System.String[] paths, System.String[] foldersToSearch, BuildTarget target) (at C:/buildslave/unity/build/Editor/Mono/AssemblyHelper.cs:192) UnityEditor.HostView:OnGUI()

I already Checked the forum for some similar topic. Changing the subset to net2.0 worked for a desktop build but as this option does't exist for webapp export i'm stucked.

Does anybody knows how to solve the problem and allows me to export it? By the way I'm using the last version of Unity(5.0.1f). Thanks! Soulwax62