Extend list of two-parameter roxygen commands

Treat font lock for @importMethodsFrom and @importClassesFrom the same
as that of @importFrom: package name (first parameter) in
variable-name-face, objects to import (second parameter) in comment-face.
Merge pull request #80 from brentonk/roxy-fontlock

Extend list of two-parameter roxygen commands: patch is "obvious"