Begin writing worker function.

Add function haddMultiple.
First working version with multiprocessing.

Now uses a pool of workers to hadd files if you have multiple cores and
multiprocessing installed. This option is configurable with -j or
--jobs.

There are still a few bugs to fix before we merge, for example if hadd
doesn't exist an unhelpful error is thrown.
Fix bug in printing.

Now correctly determine the ending file number for status messages of
the form:

    Now hadding files 41-50 of 50.

Also we now correctly respect "quite".
Add code to check for hadd.

ahadd now throws an error if "hadd" can't be found.