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

On a bit of a unit-testing kick to get comfortable with the Jetpack code, I hope this is fine! Found a couple of small issues in the media classes.

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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).