HTML styling set_precision should take subset
There's a problem in the way `.set_precision` currently works as it is being applied to all table cells indiscriminately. For instance a dataframe with means and standard errors cross validated across some parameter values the `set_precision` will easily hide a lot of detail in the results.

![screen shot 2015-11-20 at 12 03 14](https://cloud.githubusercontent.com/assets/1055719/11298401/ffdcccb6-8f7e-11e5-848c-be21bbd3eb2f.png)

The second parameter (2nd index) is not all zeros, they're just values in the range `[1e-4, 1e-3]`. Also while for the `mean` columns a precision of 2 is fine, for the standard error 4 might be more appropriate. So having a `subset=` keyword argument also for `.set_precision` would be great. 

![screen shot 2015-11-20 at 12 02 56](https://cloud.githubusercontent.com/assets/1055719/11298399/fc17f60a-8f7e-11e5-9691-58ef9c32847b.png)

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code for making the df

```
import itertools
import pandas as pd
import numpy as np

jobs = itertools.product(['a', 'b', 'c', 'd'], np.arange(1e-4, 1e-3, .0003), range(10))
rows = []
for v, v2, itr in jobs:
    rows.append({'param_1': v, 'param_2': v2, 'iter': itr,
                 'score_1': np.random.randint(0, 100, size=(1,))[0],
                 'score_2': np.random.rand(1, )[0]})
df_multi = pd.DataFrame(rows)
agg = df_multi.groupby(by=['param_1', 'param_2'])[['score_1', 'score_2']].agg(['mean', 'sem'])
```
