Engine should shut down all systems with it

Also requires a change to OgreEngine::shutdown to avoid
segfaults.
Add Bullet debug renderer
Fix Bullet install script for cross-compiling
Add cylinder collision shapes aligned to other axes
Remove background animation
Make camera follow player
Clean up player and secondEntity scripts
Allow changing Bullet's debug mode
Add Lua bindings for Ogre::Degree
Make Engine::setTargetFrameRate thread-safe
Expose BulletEngine to Lua
Tweak Bullet debug drawing
Tweak sandbox scripts
Update documentation
Make BulletToOgreSystem::update more consistent with other systems
Add Entity.id for Lua scripts
Add Entity::destroy
Fix recording of removed enitites in EntityFilter

Only record a removal if we actually had this entity before.
Fix remove of Ogre scene nodes

The OgreRemoveSceneNodeSystem needs to actually know about the
Ogre::SceneNode objects to properly remove them.
Fix SharedData to always update working copy

Previously, the working copy was only updated when it was
accessed. This lead to outdated data being copied around when
the working copy wasn't accessed in each frame.