Basic failing integration tests
Make a single edge case pass, using `ruby
test/test_months_integration.rb -n test_12_1799` to focus on it.
Making another edge case pass, while also correcting an error in my
tests.  `ruby test/test_months_integration.rb -n test_01_3001`
One more edge case.  Normally, I wouldn't commit each individual passing
test, but I wanted to emphasize here that you should tackle one test at
a time.
A basic month class
The month header now prints correctly
Zeller's Congruence.  Notice that I extracted and tested methods for the
finicky parts that were giving me trouble.  That actually took my
previously failing edge cases and made them pass!
Printing a month, for any month with 29 days.

This is not a perfect solution.  Definitely room for refactoring, which
we'll get to in a few commits.
Basic implementation of a year
Refactor is_leap_year? to be more understandable
Implement Month#length.  This brings us to the point that our month is
nearly complete.
Complete printing a single month
Refactor day padding
Year argument edge cases
Year WIP, with extra lines on certain months