Make `date` and `placeholder` attrs bindable

I think is a common use case to show an input with a placeholder without defaulting to any value. In order to achieve that, `placeholder` and `date` attributes must be binded.

Here's a simple example of how it should behave and look:

```javascript
pik-a-day type='text' placeholder='From'
pik-a-day type='text' date='' placeholder='To'
```

![screen shot 2014-11-05 at 5 11 46 pm](https://cloud.githubusercontent.com/assets/1371000/4924058/b22eccd8-651f-11e4-9234-c3730f8faa59.png)