Add module() as a union type of callback()

[mikebeam]

According to to sockjs_session:emit/2, the callback can be a module name, which is how our code uses sockjs_handler:init_state/4. The definition of the callback() type as only a fun results in a number of dialyzer warnings, which are resolved by this change.

[saurik]

The type signature of the function isn't correct anyway: it should be {ok, any()}, not just ok :/.