Cleanup of weapon fx and assets.

This reduces duplication by grouping assets used by multiple weapons in one location: shared.
This reduces duplication and cleans particle fx that was spread across multiple files.  We now have rocketfx.cs and grenadefx.cs
This also fixes some minor particle fx console warnings/errors.
Fixed missing material for Cheetah Turret.
Merge pull request #312 from thecelloman/cleanwpndir

Cleanup of weapon fx and assets.