Dataframe Representation Could be More Informative with MultiIndexes
I have been struggling a little to get my head around MultiIndexing. When playing with them in python notebook, I think it would have been much easier for me to pick things up if the `__repr__`was a little bit more informative:

For example:

``` python
print(mp_stats_unstack.__repr__())
<class 'pandas.core.frame.DataFrame'>
Int64Index: 1848 entries, 1378842548 to 1378844396
Columns: 252 entries, (0/s, 0) to (MIS/s, 3)
dtypes: float64(252)
```

With the Column attribute, the following two things were not obvious to me:
- The Column is a MultiIndex
- That the Column MultiIndex has two levels named 'var' and 'CPU'

Of course, now that I know what to look for (If a column has tuple it is (or probably is) a multiindex, I can inspect the .columns.names attribute. But when trying to grok this I think a more verbose `__repr__()` might be helpful for both learning and convenience.  
