Twenty Eleven: functions.php cleanup and introduction of theme options; see #17198

* Cleanup functions.php, adding comments and function_exists() checks following Twenty Ten's example
* Theme option for choosing an alternate (dark) color scheme. It currently only loads a placeholder CSS file with dark styles to follow.
* Theme option for selecting a link color that loads an internal style block for resetting link colors. An updated style.css will follow to take advantage of this.
* Theme options for selecting an alternate layout. Adds a class to the body element. An updated style.css will follow to take advantage of this.


git-svn-id: https://core.svn.wordpress.org/trunk@17721 1a063a9b-81f0-0310-95a4-ce76da25c4cd