ENH: Allow custom frequencies
Allows custom frequencies other than business day in testing utilities. Keeps the old default.

closes #4541

@jseabold you are closing #4541 with this! excellent!
 merge when ready
 Ok, I don't like to merge with pandas usually to make sure other eyes get on it, but I'll do this one it's simple enough. I'll edit the commit message to close that issue.
 I left custom frequencies for PeriodIndex and friends for another PR...
 gr8
