Sharing: Check if label value is set before to display it

For more details, check the original trac ticket:
http://plugins.trac.wordpress.org/ticket/1974
Jetpack: Sharing: Consolidating 'Share this' message to one location
Jetpack: Sharing: Adding sanitization function
Jetpack: Sharing: Added default label for sharing on the front end
Revert "Jetpack: Sharing: Added default label for sharing on the front end"

This reverts commit 95e05064afe5b2498cd3d5c12c22d05aafa7c116.
Merge pull request #18 from jeherve/1974-trac

Sharing: Check if label value is set before to display it