Twitter: Allow Publicize or user-defined Twitter handle used as the twitter:site meta and via.

* twitter:site enhancement
Previously, the twitter:site meta tag used in Twitter Cards is hardcoded to @Jetpack. This commit sets it to @Jetpack, if not otherwise defined, then the first Publicize Twitter connection, and finally, if set, a user-determined value.

* Twitter Site Handle setting
Setting appended to sharedaddy that would set the twitter:site above, as well as append the "via @$handle" when shared on Twitter via sharedaddy.

* Default via setting
Since the via option can be controlled easily by the user, change the default from nothing to @Jetpack.

Fixes #60.