Sharedaddy: only enqueue Javascript when Shares are displayed

Check our sharing options: if the sharing buttons are set to appear on that page, we enqueue the js
Sharedaddy: move check if sharing is enabled to its own function
Sharedaddy: only enqueue CSS if the buttons should be displayed
Sharedaddy: always enqueue scripts on themes supporting IS
Sharedaddy: align statements
Sharedaddy: add filter to change when files are enqueued
Sharedaddy: only enqueue files when IS is active
Sharedaddy: remove IS from conditional

IS already takes care of enqueuing resources if needed
Sharedaddy: change $global into $global_options

To avoid any confusion with GLOBALS
Merge pull request #954 from jeherve/fix/likes-enqueues

Sharing: only enqueue styles and scripts when necessary