Unit tests for media extractor and summary, with small bug fixes.

Using strpos for substring find for (small) efficiency bump.

Modified get_link_count in class.media-summary.php to prevent matches
of unrelated tags (abbr, address, and other relatively-unused but
still valid tags).
Handle strpos return values in expressions correctly.
Merge pull request #1048 from scotchfield/media_unit_tests

Unit tests for media extractor and summary, with small bug fixes.