Adjustments to match_on examples (cf issue #46)

Changed "exclude=" and "excludes=" to "within=".
Added a contrasts.arg argument to fill.NAs for compatibility with
model.matrix --- allows use of contrasts other than the system defined
contrasts in model.matrix.
Adjusted help page for fill.NAs to reflect new argument of contrasts.arg
errors for caliper widths that are vectors
Merge branch 'master' into issue-44
math ops: getting both matrices in the same row,col order