ENH: allow multiple rolling quantiles in one pass
Currently, `rolling_quantile()` accepts only a single float for the `quantile` argument. I find myself wanting to compute multiple quantiles over the same data. Instead of doing three calls to `rolling_quantile()`, I'd like to be able to call `rolling_quantile()` once with a sequence of floats as the `quantile` argument and get back a list of results. This has benefits both in terms of code conciseness and efficiency.

This suggested behavior would be analogous to how np.percentile works. http://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.percentile.html

Currently:

```
>>> import pandas as pd
>>> ser = pd.Series(np.array([1,2,3,4,5,6,7,8,9]))
>>> ser.rolling(window=3).quantile(quantile=0.5)
0   NaN
1   NaN
2     2
3     3
4     4
5     5
6     6
7     7
8     8
dtype: float64
```

Desired enhancement:

```
>>> ser.rolling(window=3).quantile(quantile=[0.25,0.5,1])
[0   NaN
1   NaN
2     1
3     2
4     3
5     4
6     5
7     6
8     7
dtype: float64, 0   NaN
1   NaN
2     2
3     3
4     4
5     5
6     6
7     7
8     8
dtype: float64, 0   NaN
1   NaN
2     3
3     4
4     5
5     6
6     7
7     8
8     9
dtype: float64]
```
