Cyclic GC issues
A mystery to be debugged soon:

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

arr = np.random.randn(100000, 5)

def leak():
    for i in xrange(10000):
        df = pd.DataFrame(arr.copy())
        result = df.xs(1000)
        # result = df.ix[5000]

if __name__ == '__main__':
    leak()
```
