Added parse+tests for audio, video, and source elements.

[gRegorLove]

Fixes #63, reference http://microformats.org/wiki/microformats-2-parsing#parsing_a_u-_property

Note: Test UrlTest::testReturnsUrlIfAbsolute was failing and continues to fail (PHP 5.4):

Mf2\Parser\Test\UrlTest::testReturnsUrlIfAbsolute with data set #16 ('relative add scheme host user from base', 'http://user:@www.example.com', 'server.php', 'http://user:@www.example.com/server.php')
relative add scheme host user from base
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'http://user:@www.example.com/server.php'
+'http://user@www.example.com/server.php'