Incorrect groupby displayed in IPython Notebook with 64-bit floats and multi-index
I ran into this bug when attempting some groupby-aggregates on data that had 64-bit floats.

```
_ids = np.array(range(201500000100,201500000110)*3).astype(float)
_df = pd.DataFrame({'id':_ids,
          'value_1':np.random.randint(0,10,30),
          'value_2':list('ABC')*10})
```

A simple groupby-aggregation produces the correct results, namely returning a DataFrame with an index of 10 rows.

```
_gb1 = _df.groupby('id').agg({'value_1':np.sum})
```

| value_1 | id |
| --- | --- |
| 2.015000e+11 | 14 |
| 2.015000e+11 | 4 |
| 2.015000e+11 | 8 |
| 2.015000e+11 | 25 |
| 2.015000e+11 | 12 |
| 2.015000e+11 | 16 |
| 2.015000e+11 | 15 |
| 2.015000e+11 | 21 |
| 2.015000e+11 | 8 |
| 2.015000e+11 | 7 |

However, grouping by the second value to return a multi-index returns an unexpected DataFrame. 

```
_df.groupby(['id','value_2']).agg({'value_1':np.sum})
```

| id | value_2 | value_1 |
| --- | --- | --- |
| 2.015000e+11 | A | 2 |
|  | B | 5 |
|  | C | 3 |
|  | A | 3 |
|  | B | 0 |
|  | C | 0 |
|  | A | 4 |
|  | B | 1 |
|  | C | 3 |
|  | A | 8 |
|  | B | 7 |
|  | C | 0 |
|  | A | 3 |
|  | B | 0 |
|  | C | 2 |
|  | A | 0 |
|  | B | 4 |
|  | C | 5 |
|  | A | 4 |
|  | B | 9 |
|  | C | 0 |
|  | A | 6 |
|  | B | 6 |
|  | C | 6 |
|  | A | 8 |
|  | B | 3 |
|  | C | 4 |
|  | A | 5 |
|  | B | 7 |
|  | C | 3 |

Casting the `id` variable to unicode appears to correct the problem and return an aggregated DF with the correct dimensions and multi-indexing.

```
_df['id_str'] = _df['id'].astype(unicode).str.slice(0,-2)
_df.groupby(['id_str','value_2']).agg({'value_1':np.sum})
```

---
## INSTALLED VERSIONS

commit: None
python: 2.7.10.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 69 Stepping 1, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.16.2
nose: 1.3.7
Cython: 0.22.1
numpy: 1.9.2
scipy: 0.16.0
statsmodels: 0.6.1
IPython: 3.2.1
sphinx: 1.3.1
patsy: 0.3.0
dateutil: 2.4.2
pytz: 2015.4
bottleneck: 1.0.0
tables: 3.2.0
numexpr: 2.4.3
matplotlib: 1.4.3
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: 1.0.0
xlsxwriter: 0.7.3
lxml: 3.4.4
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.5
pymysql: None
psycopg2: None
