Unlimited layout embedding

This commit removes the two layout limit. Any number of non-repeating
layouts may be used and will be rendered in order (top down).

Unlike the previous layout rendering code, this version does not double
render the layouts. The previous version would render the sub-layout,
then render the master layout embedding (without re-rendering) the
sub-layout, and then re-render the combined layout with the page
content.
Add three layout test case.
Merge pull request #211 from Stebalien/recursive-layouts

Recursive layouts