Use PHPunit DOM assertions instead of deprecations

As of PHPUnit 4.2 assertions built-in into PHPUnit about the DOM
are extracted and refactored in
https://github.com/phpunit/phpunit-dom-assertions

I've replaced `PHPUnit_Framework_TestCase` with
`PHPUnit_Framework_DOMTestCase`.

The one failing test with the new assertions was
`Jam_Form_GeneralTest::test_file`. `[value=""]` no longer matches when
the `value` attribute is missing altogether.
Merge pull request #57 from OpenBuildings/phpunit-deprecations

Use PHPunit DOM assertions instead of deprecations