Add Update, PostUpdate, Save, Load function in LuaScriptInstance, Add one more LoadXML and SaveXML in Scene, Add destructor function in Math lib.

git-svn-id: http://urho3d.googlecode.com/svn/trunk@2618 3bd170d8-ddcf-f82c-f583-7a401ccf5608
Add VehicleDemo sample in Lua, using update function in script object.

git-svn-id: http://urho3d.googlecode.com/svn/trunk@2619 3bd170d8-ddcf-f82c-f583-7a401ccf5608
Update Lua script API documention.

git-svn-id: http://urho3d.googlecode.com/svn/trunk@2620 3bd170d8-ddcf-f82c-f583-7a401ccf5608