Add TextEffect in Text3D class.

git-svn-id: http://urho3d.googlecode.com/svn/trunk@2656 3bd170d8-ddcf-f82c-f583-7a401ccf5608
Update document.

git-svn-id: http://urho3d.googlecode.com/svn/trunk@2657 3bd170d8-ddcf-f82c-f583-7a401ccf5608