Add support for multiple-line legend items in vertical (default) mode

I have modified the legend in the default vertical mode in order to:
* Add a `tspan` element to each legend item for each line in the datum's text
* Detect overflow whenever there are too many items, in which case it spawns as many columns as necessary.

For the following example, the crossfilter dimension associated with legend's elements has records which are multi-line strings (i.e. strings with `\n` characters). For example:
```
<!-- d.Dim has items like 'T1,T2_oneline; T3_anotherline' -->
ndx = crossfilter(data);
legendDim = ndx.dimension(function(d) { return d.Dim.replace(/;/g, '\n') });
```
![snapshot36](https://cloud.githubusercontent.com/assets/4201057/2609621/df8e6bd0-bb78-11e3-82db-78a4e0237f0e.png)

I hope this was clear enough. 
I'm new to this JS dev business, just installed node, grunt, and ran `grunt lint` and `grunt test`. `test` failed due to 1 single warning, I don't know what, but this code is working as per the screenshot.