Systems support

[hannobraun]

Preliminary support for systems has been added: https://github.com/hannobraun/rustecs/blob/master/rustecs/tests/systems.rs

I think this goes in the right direction, but isn't really usable yet. There are still a few things missing before the feature can be considered complete:

• An Event enum should be generated with variants for each declared event type. I'd like to pass on the event into the systems, and I think this is the best solution to achieve this in a type-safe way.
• The event object passed to trigger should be passed on to the triggered systems.
• A Control should be passed into each system. The control's apply method should be called after all systems have run.
• Usage should be documented in the README.