Site Logo: Add the WordPress.com Site Logo feature to Jetpack.

Site Logo allows a user to upload a logo that will persist and be displayed in all themes that support the feature.

WordPress.com support page: http://en.support.wordpress.com/site-logo/

Standalone plugin: https://github.com/Automattic/site-logo
Site Logo: Prefix theme functions.
Site Logos: Update the docblock for jetpack_the_site_logo().
Site Logos: Avoid JS Uncaught Errors in the Customizer when logo objects have no size properties.
Site Logo: Sanitize header text classes for output.
Merge branch 'master' of github.com:Automattic/jetpack into site-logo-escaping
Site Logo: Add a media state for site logos in the Media Manager.
Site Logo: Update the Customizer section heading to improve discoverability.

Also simplified the control name to just "Logo" (keeps it concise and matching the section heading).
Site Logo: Use the proper helper function for determining registered image sizes.

This will break on WP.com, as `get_intermediate_image_sizes()` has been neutered because of Photon. We'll deal with it over there.
Merge branch 'master' into site-logo
Site Logo: change the logo anchor class to "site-logo-link", which is more in line with core convention.
Merge branch 'site-logo' into site-logo-escaping
Site Logo: Remove unnecessary hook check, which is now handled properly by core. Props mattwiebe.
Site Logo: Properly pre-cache selectors and prevent scope leak. Props mattwiebe.
Site Logo: Remove redundant `description` property, which is already in WP_Customize_Control.
Merge branch 'master' into site-logo
Merge branch 'site-logo' into site-logo-escaping
Site Logo: Reset the logo setting if the current site logo gets deleted from the media manager.
Merge branch 'site-logo' into site-logo-escaping
Site Logo: Validate and sanitize new site logo settings.