Mutliple stacked bars per category (Morris.Bar)

This addresses issue #141. It requires the y-values be from the same domain (e.g. there should be a correlation between values types) of course. So you can't mix %'s with absolute values and so on...

The case? For instance, let's assume that we want to show Lotto results for last 4 weeks. Our data set contains:

* Percentage of players who gained £10,
* Percentage of players who gained £100,
* Percentage of players who gained £1,000,
* Percentage of players who gained £0.

Now we can, for each week, show two stacks. The first one will contain %'s of players who won anything, the other one, % of players who got none.

The usage is for cases when the result is not always B/W but we want to show the groups.

The API is backward compatible (```@options.stacked``` accepts ```true|false```) and fixes ```@ymax``` issue from the basic stacks functionality.

![multi-stacked-bars](https://f.cloud.github.com/assets/52669/36503/4d9ab0d2-5344-11e2-8a7f-498a125a548d.png)