Memory leak in resample?
Hi,

when using the resample method, additional memory is allocated which seems to remain after the call. Consider the following example:

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

# taken from http://pandas.pydata.org/pandas-docs/stable/10min.html?highlight=resample
rng = pd.date_range('1/1/2012', periods=100, freq='S')
ts = pd.Series(np.random.randint(0, 500, len(rng)), index=rng)

# make use of objgraph 
objgraph.show_growth(limit=20)
print ""
ts = ts.resample('5Min', how='sum')
ts = None
gc.collect()
objgraph.show_growth(limit=20)
```

Output on my machine (Ubuntu 14.04.1, 64bit):

```
function                      16127    +16127
tuple                          4324     +4324
dict                           4025     +4025
list                           2170     +2170
cell                           2151     +2151
weakref                        2131     +2131
wrapper_descriptor             1795     +1795
builtin_function_or_method     1635     +1635
type                           1447     +1447
getset_descriptor              1427     +1427
method_descriptor              1056     +1056
property                        780      +780
module                          641      +641
instancemethod                  581      +581
member_descriptor               469      +469
set                             335      +335
instance                        282      +282
classobj                        247      +247
cache_readonly                  167      +167
classmethod                     151      +151

wrapper_descriptor     1804        +9
getset_descriptor      1431        +4
dict                   4029        +4
member_descriptor       472        +3
function              16128        +1
Day                       1        +1
cell                   2152        +1
DatetimeEngine            1        +1
Minute                    1        +1
```

Thus, the space is still allocated for certain objects. Have I missed something? 

Module versions:
pandas: 0.14.1
scipy: 0.13.3
numpy: 1.8.1
