Sharing: made share counter wrappers use data attributes

Previously sharing counter wrapper anchor tags were using
id properties to identify themselves. Now a data attribute is
used to make inserting several share counters for one post possible.
Sharing: made the get_counts code more DRY

as well as fixed variables leaking to the global scope.
Sharing: changed wrong parameter name in the count injection method.
Merge pull request #1374 from zinigor/sharing/multiple-share-counters

Sharing: adds a possibility to use multiple share counters per post