Added valueOf for even more magic dependencies

This is interesting, but there are some pitfalls:

When comparing two observables it will compare based on object equality and may have unexpected results when two observables have the same value.

    a = Observable 5
    b = Observable 5

    computed = Observable ->
      if a == b
        ...

When an individual observable occurs as the predicate of an if statement:
    
    boolean = Observable false

    computed = ->
      if boolean # Will be evaluated as if it were true!
        ... 

These pitfalls may offset the gains from using this technique :(