Allow freelist overflow.

This commit is a backwards compatible change that allows the freelist to overflow the
page.count (uint16). It works by checking if the overflow will occur and marking the
page.count as 0xFFFF and setting the actual count to the first element of the freelist.

This approach was used because it's backwards compatible and it doesn't make sense to
change the data type of all page counts when only the freelist's page can overflow.

Fixes #192.
Remove ErrFreelistOverflow error.
Merge pull request #219 from benbjohnson/freelist-overflow

Allow freelist overflow.