Prototypal inheritance in classes

Before : 

```javascript
var parentClass = sleipnir.klass({foo:"bar"})
  , childClass = parentClass.extend({bar:"baz"})
  , childInstance = childClass.create()

childInstance.foo // "bar"
parentClass.prototype.foo = "baz"
childInstance.foo // "bar"
```

After : 

```javascript
var parentClass = sleipnir.klass({foo:"bar"})
  , childClass = parentClass.extend({bar:"baz"})
  , childInstance = childClass.create()

childInstance.foo // "bar"
parentClass.prototype.foo = "baz"
childInstance.foo // "baz"
```