Implement Lua script compiling in polybuild.

This is a basic implementation of Lua script compiling to byte-code in polybuild. I also added a prebuildActions node and an action entry under it to facilitate compilation, allowing you to do something like this:
\<PolycodeProject\>
...
   \<polyarray:prebuildActions\>
      \<item action="compileScript" source="Scripts/Main.lua" out="bc/Main.lua"/\>
   \</polyarray:prebuildActions\>

   \<polyarray:packedItems\>
      \<item type="file" source="bc/Main.lua" path="Scripts/Main.lua"/\>
   \</polyarray:packedItems\>
...
\</PolycodeProject>