fixes input update issue on keydown

This pull request fixes an issue introduced in #115: when changing a data model property bound to an input field value in the `keydown` event listener, the value is not updated in the field.
Here is a plunk to reproduce the issue: http://embed.plnkr.co/LjTFOwFr2FrQN6a7m3IG
The issue also happen in http://moneycount.herokuapp.com/ when using the up and down arrow keys on a field.

In this pull request, I am adding a check that there was a change before any synchronization (either from the data model to the field, or from the field to the data model). The reference for the comparison (to check if there was a change in the data model or in the field) is stored in the `_lastValue` property on the `EltNode` instance.