Fix root split on very large append.

This commit fixes a bug where a root split on a very large insert would
cause an overflow of the root node. The problem was that the new root
was not split when it was created so a new root with more than 64K child
nodes would overflow the page.count (uint16).
Merge pull request #228 from benbjohnson/fix-large-append

Fix root split on very large append.