More flexible symbolization pipeline

[floledermann]

Currently, choropleth maps (i.e. setting the fill attribute) are pretty much hardwired into .meta(), .legend() etc. - for the next major release there should be a more flexible way to symbolize attributes & generate appropriate legends.

This also relates to the scale system, currently we use D3's sclaes which are rather opaque meaning we can't get a lot of introspection info out of an existing scale, which makes creating inverses (for legends) and handling special values a pain. See #23 for some thoughts to consider.