allow loading of entity modules

store now provides a registerEntityModule() method to register module paths
that define entities. this is especially necessary if entities are defined in
separate modules, and module reloading is enabled: in such cases
the store might be re-initialized with only a subset or no entity constructors
at all, leading to "entity not registered" exceptions. With this change
sqlstore can now load entity modules on demand.
adapted inline docs
Merge branch 'master' into moduleloading

Conflicts:
	lib/sqlstore/store.js
removed Store.create() convenience method - saves one method call
only reload modules if there are any registered