Revert fixes involving issue #99 (which makes Arel unusable in large datasets)

There are two commits involving issue #99 which should be reverted. 

First, #99 is not an issue in Arel at all. Second, the fix provided pretty much destroys the purpose of pagination by causing full table scans.

The original code (it seems I can't simply revert the commits) is 900 times slower than this one.

/cc @tenderlove @rsim 

With love,
Eduardo

Explain plans (txt)

```
SELECT * FROM (
   SELECT raw_sql_.*, rownum raw_rnum_
   FROM (SELECT "LANCAMENTOS".* FROM "LANCAMENTOS" ) raw_sql_
 )
 WHERE raw_rnum_ between 1 and 30
 
 
----------------------------------------------------------------------------------
| Id  | Operation           | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |             |  4636K|  2701M| 23442   (2)| 00:04:42 |
|*  1 |  VIEW               |             |  4636K|  2701M| 23442   (2)| 00:04:42 |
|   2 |   COUNT             |             |       |       |            |          |
|   3 |    TABLE ACCESS FULL| LANCAMENTOS |  4636K|   738M| 23442   (2)| 00:04:42 |
-----------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - filter("RAW_RNUM_"<=30 AND "RAW_RNUM_">=1)
 
   Statistics
-----------------------------------------------------------
               4  user calls
              13  physical read total multi block requests
       202588160  physical read total bytes
       202588160  cell physical IO interconnect bytes
               0  commit cleanout failures: block lost
               0  IMU commits
               0  IMU Flushes
               0  IMU contention
               0  IMU bind flushes
               0  IMU mbu flush






SELECT * FROM (
   SELECT raw_sql_.*, rownum raw_rnum_
   FROM (SELECT "LANCAMENTOS".* FROM "LANCAMENTOS" ) raw_sql_
   WHERE rownum <= 30
 )
 WHERE raw_rnum_ >= 0
 
 
-----------------------------------------------------------------------------------
| Id  | Operation           | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |             |    30 | 18330 |     2   (0)| 00:00:01 |
|*  1 |  VIEW               |             |    30 | 18330 |     2   (0)| 00:00:01 |
|*  2 |   COUNT STOPKEY     |             |       |       |            |          |
|   3 |    TABLE ACCESS FULL| LANCAMENTOS |    30 |  5010 |     2   (0)| 00:00:01 |
-----------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - filter("RAW_RNUM_">=0)
   2 - filter(ROWNUM<=30)
 
   Statistics
-----------------------------------------------------------
               4  user calls
               0  physical read total multi block requests
               0  physical read total bytes
               0  cell physical IO interconnect bytes
               0  commit cleanout failures: block lost
               0  IMU commits
               0  IMU Flushes
               0  IMU contention
               0  IMU bind flushes
               0  IMU mbu flush
```