Legend Attributes

A new 'legend-label' attribute chould be added in order to facilitate the creation of legends from data in the simplestyle-spec format. I'm working on a libraary to take simplestyle-spec data and produce an svg legend on the fly. A legend label attribute could be of use.

```
// OPTIONAL: default ""
// text to define this object in a legend
// 
"legend-label": "Bus Stop",
```