Remove WARNING on content included from higher levels in the tree.

Removed WARNING thrown on content with ../ style relative paths. Due to
increasing use of deeper tree structures in openstack-manuals this
warning was largely reporting false positives.

Raised Issue #30 to address the need for a better common content check
in the future.
Issue #1: Prune common content of unused files pre-transform.

Perform a basic prune of content from the common directory that is not
included in any other file. This assists with transformation speeds by
minimizing the transformation of content that wont actually appear in
the book.