Better error message if non-class passed to createActions() or createStore()

[acdlite]

The problem in issue #47 could have been avoided if a better error message were thrown.

[jackcallister]

I've been having a look at this and it doesn't seem possible to question if an object is a class since it gets compiled into a function. A simple answer might be to have the 'forgot to extend store' error suggest making sure that the object is indeed a class.