Fixed Issue#3 and unit tests

[joashc]

Fixed Issue #3, and a few other issues preventing the unit tests from passing.

Before these changes, we get the You can only make functions into Conduits error, as mentioned in Issue #3.

> postal.when@0.2.0 test
> mocha ./spec/*.js
6 failing

    1) postal.when when calling when with default behavior "before all" hook:
       Error: You can only make functions into Conduits.
    2) postal.when when calling when with default behavior "after all" hook:
       TypeError: Cannot call method 'dispose' of undefined
    3) postal.when when calling when with once = true "before all" hook:
       Error: You can only make functions into Conduits.
    4) postal.when when calling when with once = true "after all" hook:
       TypeError: Cannot call method 'dispose' of undefined
    5) postal.when when calling when with a timeout that triggers onError The onError callback should be invoked:
       Error: You can only make functions into Conduits.
    6) postal.when when calling when with a timeout that does NOT trigger onError data should match expected:
       Error: You can only make functions into Conduits.

npm ERR! Test failed.  See above for more details.

After adding no-op callbacks for subscriptions with undefined callbacks, and fixing a few other minor issues, all unit tests pass:

> postal.when@0.2.0 test
> mocha ./spec/*.js

  postal.when
      when calling when with default behavior
         ✓ should invoke the callback the expected number of times 
         ✓ data_a should match expected 
         ✓ data_b should match expected 
         ✓ data_c should match expected 
         ✓ data_d should match expected 
      when calling when with once = true
         ✓ should only invoke the callback once 
         ✓ data_a should match expected 
         ✓ data_b should match expected 
         ✓ data_c should match expected 
         ✓ data_d should match expected 
      when calling when with a timeout that triggers onError
         ✓ The onError callback should be invoked 
      when calling when with a timeout that does NOT trigger onError
         ✓ data should match expected (1111ms)

12 passing (1s)