Modify calls to matrixStats functions to mitigate non-backward comptible
changes in matrixStats 1.0.0. More precisely:
- 'useNames=NA' became deprecated in matrixStats 1.0.0 so we make sure to
  never pass 'useNames=NA' when calling a matrixStats function;
- furthermore, we make sure to pass a 'useNames' value that preserves the
  pre-matrixStats 1.0.0 behavior in order to avoid disruption.
See #31 for a
discussion of these changes.

…avior

- Thanks Kurt Hornik for bringing to my attention; caught by `tools::checkRd('inst/NEWS.Rd')`.