Add Urho3D::String support in tolua++.

git-svn-id: http://urho3d.googlecode.com/svn/trunk@2571 3bd170d8-ddcf-f82c-f583-7a401ccf5608
Remove Urho3D::String binding in Lua.

git-svn-id: http://urho3d.googlecode.com/svn/trunk@2572 3bd170d8-ddcf-f82c-f583-7a401ccf5608