Filtering of data, including refreshing filtered dataset example

Hi,

We talked a while ago about being able to filter a "master" dataset into subsets for each graph using filters.

https://github.com/cmaurer/angularjs-nvd3-directives/issues/167

This PR is how I have used filters in our NG project, hence a detailed example that includes a refreshing dataset (simulating HTTP requests using $interval) and filtering a master dataset into two subset graphs.

Feedback welcome! :)