Resolvable expression can be treated as Reference, solves #319

At render time, it first tries to normalize to a keypath and later at attribute binding checks if the keypath are able in `ractive.get()`
Here is example http://jsfiddle.net/7KfEe/2/ , we can see `items` do get updated when we type in inputfield. Atleast outside the items loop but not inside, so there is still something that is missing, but it works.