Convert tests to PHPUnit

I've switched the tests over to PHPUnit. I'm using the composer autoloader as a bootstrap, so I added an install directive to the travis config for that. Tests that rely on pear/Date, which is specified as a suggested dependency in composer.json, are marked as skipped if it is not installed. Also, for now I'm setting the error_reporting level in the phpunit.xml to suppress E_DEPRECATED and E_STRICT warnings/errors that are being raised by the package itself (rather than the tests). I might try to look into fixing those.