ENH: Index to act like Series for common ops (DataFrame.index.max() crashes)
related #4056, #5202, #5502, #5519, #6027

I have a DataFrame of length 390,080, with int64 indices.
`df.index.size` gives the correct answer of 390,080, but `df.index.max` results in a huge traceback that ends with:

```
/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/common.pyc in pprint_thing(thing, _nest_lvl, escape_chars, default_escapes, quote_strings)
   2051             hasattr(thing, 'next'):
   2052         return unicode(thing)
-> 2053     elif (isinstance(thing, dict) and
   2054           _nest_lvl < get_option("display.pprint_nest_depth")):
   2055         result = _pprint_dict(thing, _nest_lvl,quote_strings=True)

RuntimeError: maximum recursion depth exceeded in __instancecheck__
```

Using pandas 0.12.
