bubble chart WIP

[gordonwoodhull]

• brought in @monostop's bubble chart code to the dc-js/dc.leaflet.js structure
• have not tested this or created an example yet
• replaces add bubble chart #12

[gordonwoodhull]

Other ideas here

• currently color is used for selection status, but it's sometimes nice to encode other data using the color
• transparency could also be used for selection status (or for other data?)
• markerChart uses locationAccessor, defaulting to keyAccessor, whereas this uses keyAccessor - I think the markerChart approach is better.

[gordonwoodhull]

Merged to master for 0.3.0. Will open issues for the other stuff.