Allow more direct access to specific template methods

With the now deprecated Template declaration syntax, you could directly access template methods:

``` javascript
Template.myTemplate.myMethod = function() {
  return 'result'
}

Template.myTemplate.myMethod() // outputs 'result'
```

With the prefered way to declare helpers, you cannot directly access them:

``` javascript
Template.myTemplate.helpers({
  myMethod: function() {
    return 'result'
  }
})

Template.myTemplate.myMethod() // outputs undefined

// you have to call the method either through a getter
Template.myTemplate.__helpers.get('myMethod')()

// or though a hack
Template.myTemplate.__helpers[' myMethod']() //notice the empty space before 'myMethod'
```

This PR would allow to call it on a omre direct way:

``` javacript
Template.myTemplate.__helpers.myMethod()
```

Is there a reason for adding the white spaces before the argument names on HelperMap.prototype.set?