Various updates and minor bug fixes

Thanks for this plugin, we use it on our internal documentation site for sharing snippets.  Made a handful of changes that I thought I'd share:

* Bringing github version of plugin into parity with WP.org plugin repository
* Fixing PHP deprecated warning: eregi() is deprecated as of PHP 5.3.0
* Use WordPress's HTTP API to get the Gist content, since that's built to be robust and handle many different server configs.
* Adding in simplistic Gist ID validation
* Add proper escaping when printing the URL to screen.
* Since the Gist URL structure is the same throughout, only set it once and add to it as needed.
* Sanitize the filename.
* Update Gist ID parsing to account for usernames in the Gist.
* Fixing off-by-one error in Gist username detection.
* Changed to show error message if id is null. Previously it would show an error if both id and file were null, but the error wouldn't output anything useful.
* Removing unnecessary trim($file), sanitize_file_name() handles this for us.
* Fixing typo in search engine fallback url
* Changing ?> to //EOF so nobody has to worry about whitespace after the closing ?>
* Bumping "Requires at least" and "Tested up to"