Fix for oEmbed cache not clearing on update.

Replaced do_shortcode() with apply_filters( 'the_content' in method delete_gist_transients in class GistPress. do_shortcode() does not does not process oEmbeds, which prevented the shortcode method from being executed and clearing the cache.
Fix for oEmbed cache not clearing on update

Replaced do_shortcode() with $GLOBALS['wp_embed']->autoembed() in method delete_gist_transients in class GistPress. do_shortcode() does not does not process oEmbeds, which prevented the shortcode method from being executed and clearing the cache.

This is an improvement over my previous pull request, which used apply_filters( 'the_content' and therefore applied signficantly more filters than necessary.  Thank you to GaryJones for this improvement.
Correct erroneous output from method rebuild_shortcode in class GistPress

Replace $rawattr with $attrs in method rebuild_shortcode in
class GistPress as per @bradyvercher's comment in
[Issue 34](https://github.com/bradyvercher/gistpress/issues/34)

Old Erroneous Behavior: [gist 7243951 1]
New Corrected Behavior: [gist id="7243951"]
Merge branch 'master' of git://github.com/salcode/gistpress into salcode-master
Add missing slash in the plugin text domain path.