Replaced all instances of get_stylesheet_directory_uri() with get_templa...

Per: http://codex.wordpress.org/Function_Reference/get_stylesheet_directory_uri

This will allow bones to be used with child themes.  Even if it does not bring 100% compatibility to child themes, it moves the ball down the field.

Five instances total exist over these files: bones.php, custom-post-type.php.

This issue fixes #549.

```
$grep -R "get_stylesheet_directory_uri" *
library/bones.php:              wp_register_script( 'bones-modernizr', get_stylesheet_directory_uri() . '/library/js/libs/modernizr.custom.min.js', array(), '2.5.3', false );
library/bones.php:              wp_register_style( 'bones-stylesheet', get_stylesheet_directory_uri() . '/library/css/style.css', array(), '', 'all' );
library/bones.php:              wp_register_style( 'bones-ie-only', get_stylesheet_directory_uri() . '/library/css/ie.css', array(), '' );
library/bones.php:              wp_register_script( 'bones-js', get_stylesheet_directory_uri() . '/library/js/scripts.js', array( 'jquery' ), '', true );
library/custom-post-type.php:                   'menu_icon' => get_stylesheet_directory_uri() . '/library/images/custom-post-icon.png', /* the icon for the custom post type menu */
```