switching requests to soundcloud to use $.ajax with jsonp

cleaned up the .parent().parent().parent().parent().parent()  stuff for finding soundcloud track links

switched $.get() requests to $.ajax with 'jsonp datatype specified to help with cross-Domain access.

Correction; they're being detected, but only on initial pageload, due to [#L59 of `script.js`](https://github.com/martindale/soundtrack.io-chrome/blob/256417c08fe14212bbefcaa8f5ea7afe878e746d/chrome-extension/script.js#L59) only being able to discover the meta property we're looking for on pageload – since soundcloud is a "single page app" that doesn't update these properties when a new "view" is loaded, we fail to detect this.