Add spec for ObservableNumber
Fix test
Observable throws error if callback is not a function
Add test for setting numeric value
Add test for setting non-numeric value
Add tests for incrementing and decrementing
Add tests for incrementing and decrementing by a given amount
Add tests for incrementing and decrementing by a non-numeric amount
Add empty implementation for ObservableNumber
Remove tests from spec for throwing type errors
Fix spec so that value is set before subsequent tests
Add ‘increment’ method to ObservableNumber
Add ‘decrement’ method to ObservableNumber
Fix call to o.decrement in spec
Allow incrementing and decrementing by a given amount