Support pie charts in Flot widget

[fmpwizard]

Flot 0.7 ships with a pie chart plugin. Enhance the Lift Flot widget to support some reasonable subset of the pie chart’s features.

See https://groups.google.com/forum/#!searchin/liftweb/flot$20pie/liftweb/okbMDE3RyL0/3d401H6MdZkJ for a discussion in the Lift group.

Migrated from lift/framework#1151