sparse resampling not working with dictionary of columns?
Hello there,

Have I said that Pandas is awesome? yes, many times ;-)

I have a question, I am working with a very large dataframe of trades, timestamped at the millisecond precision. Latest Pandas 19.2 here.

I need to resample the dataframe every `200 ms`, but given that my data spans several years and I am only interested in resampling data between `10:00 am` and `12:00 am` every day (handled by `between_time()`), using a plain `resample` will crash and burn my machine. 

Instead, I tried the `sparse resampling` shown in the http://pandas.pydata.org/pandas-docs/stable/timeseries.html#sparse-resampling, but it **fails** when i provide it with a dictionary of columns. 

Is that expected? Is it a bug?

```
import pandas as pd
import numpy as np

rng = pd.date_range('2014-1-1', periods=100, freq='D') + pd.Timedelta('1s')
ts = pd.DataFrame({'value' : range(100)}, index=rng)


from functools import partial
from pandas.tseries.frequencies import to_offset

def round(t, freq):
 freq = to_offset(freq)
 return pd.Timestamp((t.value // freq.delta.value) * freq.delta.value)

# works
ts.groupby(partial(round, freq='3T')).value.sum()

# does not work
ts.groupby(partial(round, freq='3T')).apply({'value' : 'sum'})

ts.groupby(partial(round, freq='3T')).apply({'value' : 'sum'})
Traceback (most recent call last):

  File "<ipython-input-104-6004b307a469>", line 1, in <module>
    ts.groupby(partial(round, freq='3T')).apply({'value' : 'sum'})

  File "C:\Users\m1hxb02\AppData\Local\Continuum\Anaconda2\lib\site-packages\pandas\core\groupby.py", line 674, in apply
    func = self._is_builtin_func(func)

  File "C:\Users\m1hxb02\AppData\Local\Continuum\Anaconda2\lib\site-packages\pandas\core\base.py", line 644, in _is_builtin_func
    return self._builtin_table.get(arg, arg)

TypeError: unhashable type: 'dict'

```


Problem is: I need to resample several columns at once in my dataframe, eventually using different functions (`sum`, `mean`, `max`). Is anything wrong here?

Thanks~