Changes the way Events are handled in the Visualization Library

This pull request addresses issues with the way events are currently handled in the visualization library.

Currently, in order to listen for events on the chart, the `addEvents` flag needs to be set to **true** in the params passed to the chart upon initialization. Given that this is not an ideal way to handle events, a refactoring of the code was in order.

With this pull request, events can now be added to the **Vis** object, and they will be bound to each chart. There is no need to set an `addEvents` flag. For example:

```javascript
// initialize chart with DOM element and vislib params
var chart = new Vis($el, vislibParams);

// add events and listeners to the chart(s)
chart.on('click', function (e) {
  console.log(e);
});
```

This is all you will need to do in order to add events to charts. You will also be able to add multiple listeners for each event. You can also add events to charts before the `render` method is called as well as after. For example:

```javascript
// add events before the render method
chart.on('click', function (e) {
  console.log(e);
});

// render chart
chart.render(data);

// add events with listener after chart(s) are rendered
chart.on('brush', function (e) {
  console.log('brushing');
});
```

This will cause both `'click'` and `'brush'` events to be bound to the chart(s). 

To turn off events, you have two options.

1. Remove a listener from an event array
2. Remove the event all together

For example:

```javascript
// 1. Remove a listener from an event array
chart.off('click', function (e) {
  console.log(e);
});

// 2. Remove the event all together
chart.off('click');
```

In example 1, if the `click` array has several listeners (i.e. the array !== 0), then only the specified listener is removed. The chart still listens for click events and emits the event to the listeners in the array. However, if the `array.length === 0`, then the event is removed from the chart.

In example 2, the 'click' event is removed all together from the chart. Events can be removed both before and after charts have been rendered.

Changes were made in several places in the code base to accommodate the changes to the way events are handled. Changes to the parts described above are found in the `vis.js` and the `handler.js` files.

In order to reduce duplicate code in the **visualization modules**, the visualization event code was consolidated into the `dispatch.js` class. This move required a change in the way we were handling styling changes with events, as each chart required slightly different styling for events. The css styling was moved out of javascript and into the less files. And since the less files were a mess and disorganized, they were refactored and broken up into smaller, more manageable files.

Finally, tests were written for the `on` and `off` methods in `vis.js` and for the `enable` and `disable` methods in `handler.js`. Tests still need to be written for `dispatch.js` methods as well as all **visualization** modules.