Further julian code cleanups
Move julian tests to TAP

I anticipate that library(julian) will eventually have a huge test
suite.  Using TAP lets me run many tests in parallel and to generate
pleasant HTML test results for publication.
Tests and extra goodness for library(julian)
Rename weekday to dow

The old name means "day of the week besides Sunday or Saturday" which
isn't actually what I meant.  I meant "day of the week".
Unfortunately, that is really long so I'm settling on the cryptic
"dow".  I can change this later if it proves foolish.  I could also
add day_of_week as an alias, if that makes better sense.