Resample yields empty groups
With some parameters, the last group yield by resample is empty. Example : 

``` python
import pandas as pd
import numpy as np

df = pd.DataFrame(np.random.normal(size=(10000,4)))
df.index = pd.timedelta_range(start='0s', periods=10000, freq='3906250n')

df.loc['1s':,:].resample('3s',how=lambda x : len(x))
```

Depending of the 'how' function used, this can lead to surprising bugs.

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.8.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-46-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8

pandas: 0.16.0-294-g45f69cd
nose: 1.3.6
Cython: 0.20.2
numpy: 1.9.2
scipy: 0.14.0
statsmodels: 0.5.0
IPython: 3.0.0-dev
sphinx: 1.2.2
patsy: 0.3.0
dateutil: 2.4.2
pytz: 2015.4
bottleneck: None
tables: 3.1.1
numexpr: 2.4
matplotlib: 1.4.3
openpyxl: 1.7.0
xlrd: 0.9.2
xlwt: 0.7.5
xlsxwriter: None
lxml: None
bs4: 4.3.2
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 0.8.2
pymysql: None
psycopg2: 2.5.3 (dt dec mx pq3 ext)
```
