Fixed leak in ofxLua::exists() -- Lua stack grew each time it was called

- also added local debugging class LuaDebugStack, which traces the Lua stack
  size when it comes into and leaves scope.  To use, add this at the top
  of a method, or at any level of scope you want to examine:
      LuaDebugStack debug(L, "exists()");
Merge pull request #7 from totalgee/master

Fixed leak in ofxLua::exists() -- Lua stack grew each time it was called