Add "Shares" on a Post support deserializing share count

As noted in this thread (https://groups.google.com/forum/?fromgroups=#!topic/restfb/7iVEHVIHKUg) FB is including the number of shares on a Post in the returned JSON, like:

graph.facebook.com/263397257076312_334940939949675:

<snip>
  "shares": {
        "count": 1
      }, 

It doesn't appear to be fully documented on the Post api page but I've consistently seen the count data (but not the actual shares). The pull request adds a method to Post to getShareCount and will deserialize the json into the Post object.

I've manually tested with Posts that have shares and do not have shares without issue.