A way to let users define custom made bound {{if}} statements

This pull requests makes it possible to create bound {{if}} statements more easily.

Here's a use case:

I'm working on a project where I'm displaying amount of rain that fell within the last hour.
These rainmeters are very fickle however, and don't always send their data to my server.
So when the server doesn't have any data it sends back: {rainfall: null}

Now when the users rainmeter doesn't send data I want the user to know so I show a message:

``` handlebars
{{if rainmeter.rainfall}}
  {{rainmeter.rainfall}}mm
{{else}}
 No Data
{{/if}}
```
Now here comes the problem sometimes the weather outside is delightfull and no rain falls.
So the value {{rainfall: 0}} must still be displayed as rainfall: 0 mm.

So I created this helper which only says 'undefined' or 'null' is false but not the number zero:

``` javascript
Handlebars.registerHelper("ifData", function(property, fn)
{
	var context = (fn.contexts && fn.contexts[0]) || this;

	var func = function(result)
	{
		if (typeof result === 'boolean')
		{
			return result;
		}

	    return result != undefined && result != null;
	};

	return Ember.Handlebars.helpers._bind.call(context, property, fn, true, func, func);
});
``` 

This didn't work because I cannot call the 'bind' function in Ember.Handlebars.helper.

This pull simply makes it available as _bind so other helpers can use it to.

Now I can use:

``` handlebars
{{ifData rainmeter.rainfall}}
  {{rainmeter.rainfall}}mm
{{else}}
 No Data
{{/ifData}}
```
And it is bindings aware so when rainfall changes so does the template.