Add switch class to SSL.
Bridge gravity setting to mruby scripting API
Move gravity handling up to cMoving_Sprite.

It was there previously, but Update_Gravity() was first implemented
by cEnemy. As other things may fall as well, this did not make
really sense.
First implementation of crate
Make falling crates kill enemies.
Properly set and use CRATE_SLIDE.

Crates may be moving due to other reasons than hitting (e.g.
standing on a moving platform). In those cases they of course
should not be slowed down.
Make crates available to mruby scripting interface
Merge branch 'crate' into devel. Closes #64.