Fix last element seek.

This commit fixes a bug with seeks where seeking to a key between the end
of a page and the beginning of the next page causes it to return nil. This
was fixed by calling Cursor.next() when the cursor ends up between pages.

This also changes Cursor.Next() so that calling Next() at the end of a bucket
leaves the cursor at the end instead of zeroing out the stack.

Fixes #186.

/cc @PreetamJinka
Add seek forward test.
Refactor Cursor.Next() to use Cursor.next().
Merge pull request #187 from benbjohnson/fix-seek

Fix in-between page seek