Revert issue #99 fixes

There are two commits involving issue #99 which should be reverted. 

First of all, #99 is not an issue in Arel at all. Second, the fix provides pretty much destroys the purpose of pagination by cause full table scans.

The original code (it seems I can't simply revert the commits) is 900 times slower than this one.

You can find the explain plans and times here:

https://gist.github.com/eduardordm/5172519