Added ability to set Observe property to undefined

The alternative is to use removeAttr, but this requires knowing that you want to remove it. Some times it is nice to set an attribute to a value without first checking the definedness of the value. First example.

```javascript
state.attr('foo', someFunction()); // someFunction could return anything.
```

Fixes #67
Merge pull request #414 from matthewp/set-property-undefined

Added ability to set Observe property to undefined