Allow multiple effects in an effect module

[einarf]

A pretty clear feature request from several people:

The effect module should probably be renamed to effects and allow for multiple effect definitions. This gives the users a lot more freedom. If they want all their effects in the same module, you should be able to do that.

[einarf]

This means an effect must be identified with testdemo.myeffect.effects.MyEffectClass. The difference is that the class is included.

A fallback when the class is not specified: Pick the first effect in the module. Also, we should probably support both effect and effects with some deprecation message for the old version.

[einarf]

Done in the 2.0 branch