Merge pull request #54 from oreillymedia/sarah

Adding logic for callouts that lack @href attributes
Fixed couple bugs in ulink handling: 1) attributes were being propagated to hyperlink text nodes when source ulink text node was empty, and 2) ulinks were being considered "empty" if they had no text node children, rather than no node children at all. Added corresponding XSpec tests.
Merge pull request #55 from oreillymedia/urls

Fixed couple bugs in ulink handling
adding handling for table footnotes