Enhanced tag_cloud_data.json

This pull request makes some additions to the tag_cloud_data.json output file. For each tag object in the file, it adds a 'posts' object, which contains an array of a subset of the posts metadata associated with that tag which looks like:
    {'title':<post title>, 'url': <post url>, 'date':<post date formatted for display>, 'isodate':<post date in iso format>}

I did this to be able to create a more advanced client side display of the tags for a theme I am working on. I kept the amount of metadata to a minimum to not create a ton of overhead in getting the file.

Just realized I forgot to update the changes.txt, will update that shortly.