Stricter testing of 'monotocity' when reindexing with ffill or bfill.

Reversing an index and forward filling succeeds but produces misleading data:

```python
>>> dr = pd.date_range('2013-08-01', periods=6, freq='B')
>>> df = pd.DataFrame(np.random.randn(6,1), index=dr, columns=list('A'))
>>> df['A'][3] = np.nan
>>> df.reindex(df.index[::-1], method='ffill')
                   A
2013-08-08  2.127302
2013-08-07       NaN
2013-08-06       NaN
2013-08-05       NaN
2013-08-02       NaN
2013-08-01       NaN
```

This change enforces "monotocity" (well actually weakly increasing) and causes a ValueError to be raised.