updating the class binding to not remove existing classes

When using `bind-class` batman clears all previous classes from a node. This will make it so that when using `bind-class` the new classes will instead get added to the existing list of classes.

before:
```
# given a fish with the type 'redfish'
@accessor 'fishtype', -> @get('fish.type')

<div class='onefish twofish' data-bind-class='fishtype'></div>
<div class='redfish' data-bind-class='fishtype'></div>
```

after:
```
# given a fish with the type 'redfish'
@accessor 'fishtype', -> @get('fish.type')

<div class='onefish twofish' data-bind-class='fishtype'></div>
<div class='onefish twofish redfish' data-bind-class='fishtype'></div>
```

This will also make sure that if a class already exists on the node it wont get re-applied:
```
# given a fish with the type 'redfish'
@accessor 'fishtype', -> @get('fish.type')

<div class='onefish twofish redfish' data-bind-class='fishtype'></div>
<div class='onefish twofish redfish' data-bind-class='fishtype'></div>
```

This will also allow the addition of multiple classes via Set's, Hash's, and Object's as before, while preserving existing classes on a node.