Tidy pack file
Add development config file
Factor library out of "tomorrow" project

library(julian) started as a quick sketch of an idea that I wrote down
in a Gist: https://gist.github.com/mndrix/5173377

I then fleshed out the idea in my "tomorrow" project where I needed some
decent tools for working with dates and times:
https://github.com/mndrix/tomorrow

It's grown up enough that it's time to move out of the house and take on
the world all by itself.
Slight README touch up
Don't export mjd/1 nor nano/1

These predicates are internal implementation details.  I can expose them
later if it proves necessary, but tomorrow didn't use them and I can't
think of any practical uses outside of library(julian).
Move dow_number/2 to julian/util

This predicate is not part of the main julian interface so it shouldn't
be exported from library(julian).  However, it's useful enough that
others are likely to want access to it.  These kinds of low-level
predicates are now housed in library(julian/util) where they can be used
independently of library(julian).
fixup dow_number
Separate module for gregorian calendar predicates

Both gregorian/3 and month_number/2 are useful predicates in their own
right.  Even users who don't want to use library(julian) might want
access to these predicates.

It also opens the way for other people to create packs that define
non-Gregorian calendars.  That's proven useful in other date time
libraries.
Minor documentation cleanups