Add me maybe

This is an optimization and bug-fix release that is compatible with 0.12.x. We add two new features: Semigroup.maybePlus[T](t: T, o: Option[T]): T and Aggregator.numericSum to convert to double and and sum from any scala.math.Numeric.

The full change log is below. Thanks to all contributors!

• Optimize CMS.create(Seq[K]) #537
• Add sumOption support to primitive Rings #538
• Add Aggregator.numericSum function #539
• Add code coverage checks #541
• Add Semigroup.maybePlus for combining a value with an optional value. #544
• Clean up compilation warnings #546
• Fix algebird-spark to work with spark 2 (#550)