Travis CI: site documentation update at 2014-09-23 13:42:53 UTC.

Commit: https://github.com/urho3d/Urho3D/commit/70e7cb2d25db47f76d94a585e2e8e28cd96d9a24

Message: Give more verbose message when SOURCE_FILES variable is found not set.
Minor documentation update to stress that most of the build options supported by Urho3D project can also be pased to build script for configuring external project.
Add initial documentation on 'scaffolding' rake task.