Rectify gereneric parameter names between SparkContext and AccumulablePa...

AccumulableParam gave its generic parameters as 'R, T', whereas SparkContext labeled them 'T, R'.

Trivial, but really confusing.

I resolved this in favor of AccumulableParam, because it seemed to have some logic for its names.  I also extended this minimal, but at least present, justification into the SparkContext comments.

Author: Nathan Kronenfeld <nkronenfeld@oculusinfo.com>

Closes #2637 from nkronenfeld/accumulators and squashes the following commits:

98d6b74 [Nathan Kronenfeld] Rectify gereneric parameter names between SparkContext and AccumulableParam