Don't prevent an update if the value doesn't change

[shashi]

This is necessary to get Knockout to work properly if I understand correctly. cc @piever

This makes JuliaGizmos/Vue.jl#10 necessary. We should release them both in tandem.

closes #39 against @JobJob's will?? :-P Let me know if you still think we really need this check here! My reasoning is here

[piever]

I think Knockout can somehow be patched to workaround this (like in Vue.jl, by always making a copy of the whole object that's synced between Knockout and WebIO) but I somehow feel it's strange to have different semantics between WebIO Observables and Julia Observables (especially as there is no way to forcefully "trigger" an update right now).

[JobJob]

I think all the arguments in favour of this seem sensible.

I have a nagging feeling that there was a case that made me want to use the "don't update unless changed" semantic, but since I can't think of it, we should merge this.