Pandas quantile function very slow
The quantile function is almost 10 000 times slower than the equivalent percentile function in numpy. See code below:

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

q = np.array([0.1,0.4,0.6,0.9])
data = np.random.randn(10000, 4)
df = pd.DataFrame(data, columns=['a', 'b', 'c', 'd'])
time1 = time.time()
pandas_quantiles = df.quantile(q, axis=1)
time2 = time.time()
print 'Pandas took %0.3f ms' % ((time2-time1)*1000.0)

time1 = time.time()
numpy_quantiles = np.percentile(data, q*100, axis=1)
time2 = time.time()
print 'Numpy took %0.3f ms' % ((time2-time1)*1000.0)

print (pandas_quantiles.values == numpy_quantiles).all()
# Output:
# Pandas took 15337.531 ms
# Numpy took 1.653 ms
# True
```
