Add pushToStack() and popFromStack() methods to allow async Observations

[BigAB]

Resolves #51

When there is no func value passed to an Observation, the methods
pushToStack() and popFromStack() can be used to make observations outside
of the context of a function call.

I should add, as a bit of a constraint, Justin suggested I should avoid changing the code in start() or update(), because they are optimized and I wouldn't want to wreck performance that is already in place to add a new feature.

[justinbmeyer]

Please use ES5.

[BigAB]

Whoops, sorry, force of habit

[justinbmeyer]

why is start needed?

[justinbmeyer]

does this function need to be re-generated? Could only one be created and just use a variable?

[BigAB]

start is needed because it is called in update(), and if I modified update to not call start(), I wasn't getting updates. There may be a better way to do this though, I am open to suggestion.

I'll declare the function outside of the constructor and reuse it.

[BigAB]

An alternative to inserting the start method there, could be modifying update() to be:

	update: function(batchNum){
		if(this.needsUpdate) {
			remaining.updates--;
		}
		this.needsUpdate = false;
		if(this.bound) {
			// Keep the old value.
			var oldValue = this.value;
			this.oldValue = null;
			// Get the new value and register this event handler to any new observables.
			if (this.func) {
				this.start();
			}
			if(oldValue !== this.value || !this.func) {
				this.compute.updater(this.value, oldValue, batchNum);
				return true;
			}
		}
	},

Would that be a more preferable approach?

[BigAB]

This is repeated but I wanted it out where it could be seen

An alternative to inserting the start method there, could be modifying update() to be:

	update: function(batchNum){
		if(this.needsUpdate) {
			remaining.updates--;
		}
		this.needsUpdate = false;
		if(this.bound) {
			// Keep the old value.
			var oldValue = this.value;
			this.oldValue = null;
			// Get the new value and register this event handler to any new observables.
			if (this.func) {
				this.start();
			}
			if(oldValue !== this.value || !this.func) {
				this.compute.updater(this.value, oldValue, batchNum);
				return true;
			}
		}
	},

Would that be a more preferable approach?

[BigAB]

We decided on a call that this was not the best way to expose a way to resolve the issue.

Rather than this way, I'll create a new PR just exposing some of the lower level stateful parts of can-observation, so that one could subclass (or whatever) in their application code