Encode URLs in sharing when possible in Ghost

[evanwon]

Currently, the social sharing buttons only have access to unencoded URLs from Ghost. URL Encoding has been added to the in-development version of Ghost (https://github.com/halfdan/Ghost/blob/master/core/server/helpers/index.js - Look for encode), but it isn't yet released as of Ghost 0.3. When available, use this to encode URLs if possible.