Add light system

See issue #20
Fix initialization of MovableComponent::m_velocity

Ogre::Vector3's default constructor leaves the coordinates
uninitialized. Initialize it to Ogre::Vector3::ZERO instead.
Add entities with prefab meshes

Mostly useful for quick debugging
Add setRange function for OgreLightComponent

OgreLightComponent::Properties::setRange is convenient when you
don't want to figure out the coefficients.
Remove default light

With scriptable lights, we don't need it anymore.
Remove debug output
Add OgreLightComponent to script initializer
Restructure Lua script files

Having separate files makes more sense and demonstrates how to
load multiple files with the manifest.txt
Add missing documentation of OgreSceneNodeComponent
Merge pull request #21 from Revolutionary-Games/lights

Scriptable Lights