Fixed a few doctests
Converted to unit tests

Now uses a class to wrap up all test methods as separately-runnable
tests that each start from a clean instance prepared by running the
setUp method.
Ported tests and fixed bug

Converted tests of TOA reading; tests revealed a bug in the handling of
TOAs without DDM values. Fixed.
Fix minor bugs
Check for current version of tempo2

My version of the general2 plugin doesn't support certain options, so it
simply doesn't produce those outputs. This should not appear to be a
test failure for pint.
Updated instructions

Noted that PINT needs TEMPO and TEMPO2 for testing, and that it needs
the PINT and TEMPO environment variables to be set.