ordered_merge output out of order
``` python
In [31]: df1
Out[31]: 
                 date       val
0 2012-01-01 00:00:00 -0.199984
1 2012-01-04 00:00:00  1.175552
2 2012-01-07 00:00:00  0.190748
3 2012-01-10 00:00:00  0.993547
4 2012-01-13 00:00:00  0.582836

In [32]: df2
Out[32]: 
                 date       val
0 2012-01-02 00:00:00  0.701435
1 2012-01-05 00:00:00  0.825832
2 2012-01-08 00:00:00  0.298673
3 2012-01-11 00:00:00  1.554882
4 2012-01-14 00:00:00  0.990432

In [33]: pd.ordered_merge(df1, df2)
Out[33]: 
                 date       val
0 2012-01-13 00:00:00  0.582836
1 2012-01-14 00:00:00  0.990432
2 2012-01-04 00:00:00  1.175552
3 2012-01-05 00:00:00  0.825832
4 2012-01-07 00:00:00  0.190748
5 2012-01-08 00:00:00  0.298673
6 2012-01-10 00:00:00  0.993547
7 2012-01-11 00:00:00  1.554882
8 2012-01-01 00:00:00 -0.199984
9 2012-01-02 00:00:00  0.701435
```
