[fixes #210] adding support for `abstract` classes

This PR adds support for abstract classes, marking them with a flag:

```
/**
 * AxisType is an abstract class that manages the data for an axis.
 *
 * @module charts
 * @class AxisType
 * @constructor
 * @abstract
 * @extends Axis
 */
Y.AxisType = Y.Base.create("baseAxis", Y.Axis, [], {
});
```