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
New Jetpack_Admin class

Create empty Jetpack_Admin class and include it.
Swap out new admin_menu function
Set up temp function forwards

Have admin_page just forward to Jetpack class for the moment.
Set up $this->jetpack and use
Set up modules menu item.

Also add in a bunch of function forwards.
Swap out function redirects for contents.
Add jetpack-modules.css

Originally developed in
https://github.com/georgestephanis/jetpack-modules
Add class.jetpack-modules-list-table.php

Originally developed in
https://github.com/georgestephanis/jetpack-modules
Add fix_redirect and handler for bulk activate and bulk deactivate.

Originally developed in
https://github.com/georgestephanis/jetpack-modules
Enqueue jetpack-modules.css
Rough in admin_page_top and admin_page_bottom slugs.
Fill in admin_page_modules contents with list class.
Add JS files and update the paths to them.
Simplify the table

Drop the icon column, tags column, and description column.
Clear out a lot of legacy css
Bump the checkbox column width
Switch to a blank jetpack-admin.css
Only bring in the jetpack-modules.css when on that page.
Check for available instead of not unavailable.