Youtube functions: add jetpack namespace and move into the lib/ folder

In [this thread](http://wordpress.org/support/topic/cant-reactivate-jetpack?replies=3), the user reported conflicts between some of Jetpack's Youtube helper functions and their theme.

I figured we could prefix our functions to avoid any conflicts. I also took the opportunity to move these functions into the `lib/` folder, and I removed the `rawurlencode_deep` function that is part of WordPress since 3.4.

Problem is, by changing `get_youtube_id` to `jetpack_get_youtube_id` I broke the Media extractor ([#](https://github.com/Automattic/jetpack/blob/2.7/class.media-extractor.php#L19)):
```php
/ For these, we try to extract some data from the shortcode, rather than just recording its presence (which we do for all)
// There should be a function get_{shortcode}_id( $atts ) or static method SomethingShortcode::get_{shortcode}_id( $atts ) for these.
private static $KEEPER_SHORTCODES = array(
       'youtube',
        'vimeo',
        'hulu',
        'ted',
        'audio',
        'wpvideo',
);
```

How could I fix this issue?

Thanks!