Add Zoom to Bubble Chart

[jamesleesaunders]

This request is to add zoom to the d3-ez Bubble Chart.

This should be done by adding zoom() and zoomed() functions to the file/src/chart/bubbleChart.js.

Following the similar pattern of zoom code which has already been added to /src/chart/lineChart.js except for the bubble chart we DO want to zoom on both the X and Y axis, however upon zooming the bubbles need to maintain the same size so their radius remains constant with the legend.

A clip path may also need adding to ensure the bubbles do not bleed outside the axis.

Bubble colours and bubble stroke must also remain the same on zoom.

[jamesleesaunders]

Example:
http://bl.ocks.org/jgbos/9752277