Frequency strings don't work with loffset parameter in resampling
```

In [7]: ts
Out[7]: 
2000-01-01 00:00:00   -1.135382
2000-01-01 00:01:00    1.295421
2000-01-01 00:02:00    0.625706
2000-01-01 00:03:00    0.235525
2000-01-01 00:04:00    0.254634
2000-01-01 00:05:00   -0.393107
2000-01-01 00:06:00   -1.881196
2000-01-01 00:07:00   -2.447050
2000-01-01 00:08:00    1.055716
2000-01-01 00:09:00    1.156710
2000-01-01 00:10:00   -1.108877
2000-01-01 00:11:00   -0.710064
Freq: T

In [8]: ts.resample('5min', loffset=datetools.Second(-1))
Out[8]: 
1999-12-31 23:59:59   -1.135382
2000-01-01 00:04:59    0.403636
2000-01-01 00:09:59   -0.644939
2000-01-01 00:14:59   -0.710064
Freq: 5T

In [9]: ts.resample('5min', loffset='1min')
Out[9]: 
2000-01-01 00:00:00   -1.135382
2000-01-01 00:05:00    0.403636
2000-01-01 00:10:00   -0.644939
2000-01-01 00:15:00   -0.710064
Freq: 5T
```
