Add ability to use a function for maxClusterRadius

I find it's necessary to use a different radius at different zoom levels. This patch lets you continue to use a single number for all zoom levels, but also lets you use a function to vary the radius for each zoom level.
Update README.md

added reference to the maxClusterRadius function ability.
fixed tabs and spacing
fixed other style issues
Merge pull request #298 from Schwanksta/patch-1

Add ability to use a function for maxClusterRadius