Use action creators inside when clauses

[tomvej]

See #8 . The interface should be the following:

when('text', actionCreator, ...params);

and optionally:

when('text', [actionCreator, ...params], [actionCreator, ...params], ...);

They can be recognized by Array.isArray(). Still, I am considering leaving out the latter.

[tomvej]

Mostly works. There is still one small problem: Tests. Since when internally uses skipDependentTests from mochaHelpers -- and I want to test it is called when an error occurs. In order to do so, I'd have either to:

1. Mock skipDependentTests which I cannot do without exporting it within object.
2. Make skipDependentTests a parameter of an internal implementation of when.

[tomvej]

I am going to close this issue and do something with tests in #12