pd.DataFrame.describe percentile string precision
When using `pd.DataFrame.describe`, if your percentiles are different only at the 4th decimal place, a `ValueError` is thrown because the the percentiles that vary at the 4th decimal place become the same value.

```
In [1]: s = Series(np.random.randn(10))

In [2]: s.describe()
Out[2]: 
count    10.000000
mean      0.291571
std       1.057143
min      -1.453547
25%      -0.614614
50%       0.637435
75%       0.968905
max       1.823964
dtype: float64

In [3]: s.describe(percentiles=[0.0001, 0.0005, 0.001, 0.999, 0.9995, 0.9999])
Out[3]: 
count     10.000000
mean       0.291571
std        1.057143
min       -1.453547
0.0%      -1.453107
0.1%      -1.451348
0.1%      -1.449149
50%        0.637435
99.9%      1.817201
100.0%     1.820583
100.0%     1.823288
max        1.823964
dtype: float64
```
