Enabling HTTPS for the Tumblr sharing button

The Tumblr image can be served over https, so this change 
uses the http() method to use https when the page uses https.
Changed 4 hard coded links to protocol agnostic

In four places, links were hard coded to http
where the resources were also available over
https. Those links were changed to protocol
agnostic links in order to seamlessly use
the appropriate protocol.
Merge pull request #246 from enkrates/sharedaddy-module-ssl

Enabling HTTPS for the Tumblr sharing button