Rename jetpack_get_youtube_id to jetpack_shortcode_get_youtube_id
Renaming function back to original in lieu of a helper function.

This reverts commit 0c15abc14f7ea4a95c5a4145ff3de590556be8ba.
Add jetpack_shortcode_get_youtube_id helper function.
Merge pull request #936 from kraftbj/youtube-shortcode

Rename jetpack_get_youtube_id to jetpack_shortcode_get_youtube_id