Arithmetic by DataFrame index
I encountered a problem with doing any arythmetic from index, in other words, when a index is time (datetime64) and i would like to count something by it, i have no other option than to assign it to some column in dataframe object.
import pandas as pd

```
import pandas as pd

rng = pd.date_range('1/1/2011', periods=4, freq='H')
ts = pd.Series(rng, index=rng)

print "Data:"
print ts

print "\nSubstraction from column"
print ts-ts[0]

print "\nIndex to column"
ts['lol']=ts.index
print ts['lol']-ts['lol'][0]

print "\nSubstraction by index"
df = ts.index
print df-df[0]
```

result:

```
Data:
2011-01-01 00:00:00   2011-01-01 00:00:00
2011-01-01 01:00:00   2011-01-01 01:00:00
2011-01-01 02:00:00   2011-01-01 02:00:00
2011-01-01 03:00:00   2011-01-01 03:00:00
Freq: H, dtype: datetime64[ns]

Substraction from column
2011-01-01 00:00:00   00:00:00
2011-01-01 01:00:00   01:00:00
2011-01-01 02:00:00   02:00:00
2011-01-01 03:00:00   03:00:00
Freq: H, dtype: timedelta64[ns]

Index to column
lol   00:00:00
lol   01:00:00
lol   02:00:00
lol   03:00:00
dtype: timedelta64[ns]

Substraction by index
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-146-5a8539747b5a> in <module>()
     13 print "\nSubstraction by index"
     14 df = ts.index
---> 15 print df-df[0]
     16 

C:\winpy\WinPython-64bit-2.7.6.4\python-2.7.6.amd64\lib\site-packages\pandas\core\index.pyc in __sub__(self, other)
    853 
    854     def __sub__(self, other):
--> 855         return self.diff(other)
    856 
    857     def __and__(self, other):

C:\winpy\WinPython-64bit-2.7.6.4\python-2.7.6.amd64\lib\site-packages\pandas\core\index.pyc in diff(self, other)
    981 
    982         if not hasattr(other, '__iter__'):
--> 983             raise TypeError('Input must be iterable!')
    984 
    985         if self.equals(other):

TypeError: Input must be iterable!
```

Maybe it's just conceptional problem, but if i want to make something with date index i have to keep additional column (with the same values as index!).
When it comes to huge datasets this can be a problem, because i have to store the same thing twice, or make additional column for calculations, which is not better.
