Add the start of a custom gravity affector
Implement Gravity particle affector effects
Merge remote-tracking branch 'zini/master' into nifogre
Don't create entities and particles when only the skeleton base is needed
Start a Nif-style particle emitter

Not complete yet (doesn't handle the vertical or horizontal direction/angle),
and should probably be renamed.
Handle the vertical and horizontal parameters of Nif particles

Note that 'horizontal' is mapped to rotate around the Z axis, not Y. The Nif
particle parameters seem to be set up to expect a normal OpenGL (Direct3D?)
orientation, rather than the 90-degree pitch offset of the game.
Keep particles in local space
Handle NiParticleColorModifier with Ogre's ColourInterpolator

Note that Ogre's ColourInterpolator has a maximum of six color stages. This
seems to be fine so far, but if we want anything better, we'll need a custom
affector for it.
Fix particle direction