Add support for filtering posts by tag

Add query param 'tags'. Param accepts a comma seperated array of tag id's (result will be posts with ANY of
those ids).
Optionally tags[any] or tags[all] can be used to search specify if search
is for ALL tags or ANY of the tags.
Get posts with similar tags for related posts block
Don't show related posts if post has no tags
Fixing coding style in Posts Controller

Replace ( ! with (!
Merge pull request #209 from ushahidi/related-posts

Show similar tagged posts in related posts