BUG: Series ops with a rhs of a Timestamp raising exception (#2898)

- Series ops with a rhs of a Timestamp was throwing an exception (#2898)
- fixed issue in _index.convert_scalar where the rhs was a non-scalar, lhs was dtype of M8[ns], and was trying
  to convert to a scalar (but didn't need conversion)
- added some utilities in tslib.pyx, inference.pyx to detect and convert timedelta/timedelta64
- added timedelta64 to checknull routines, representing timedelta64 as NaT
- aded setitem support to set NaT via np.nan (analagously to datetime64)
- added ability for Series to detect and set a ``timedelta64[ns]`` dtype (if all passed objects are timedelta like)
- added correct printing of timedelta64[ns] (looks like py3.3 changed the default for str(x), so rolled a new one)

```
In [149]: from datetime import datetime, timedelta

In [150]: s  = Series(date_range('2012-1-1', periods=3, freq='D'))

In [151]: td = Series([ timedelta(days=i) for i in range(3) ])

In [152]: df = DataFrame(dict(A = s, B = td))

In [153]: df
Out[153]: 
                    A                B
0 2012-01-01 00:00:00          0:00:00
1 2012-01-02 00:00:00   1 day, 0:00:00
2 2012-01-03 00:00:00  2 days, 0:00:00

In [154]: df['C'] = df['A'] + df['B']

In [155]: df
Out[155]: 
                    A                B                   C
0 2012-01-01 00:00:00          0:00:00 2012-01-01 00:00:00
1 2012-01-02 00:00:00   1 day, 0:00:00 2012-01-03 00:00:00
2 2012-01-03 00:00:00  2 days, 0:00:00 2012-01-05 00:00:00

In [156]: df.dtypes
Out[156]: 
A     datetime64[ns]
B    timedelta64[ns]
C     datetime64[ns]
Dtype: object

In [60]: s - s.max()
Out[60]: 
0    -2 days, 0:00:00
1     -1 day, 0:00:00
2             0:00:00
Dtype: timedelta64[ns]

In [61]: s - datetime(2011,1,1,3,5)
Out[61]: 
0    364 days, 20:55:00
1    365 days, 20:55:00
2    366 days, 20:55:00
Dtype: timedelta64[ns]

In [62]: s + timedelta(minutes=5)
Out[62]: 
0   2012-01-01 00:05:00
1   2012-01-02 00:05:00
2   2012-01-03 00:05:00
Dtype: datetime64[ns]

In [160]: y = s - s.shift()

In [161]: y
Out[161]: 
0              NaT
1   1 day, 0:00:00
2   1 day, 0:00:00
Dtype: timedelta64[ns]

The can be set to NaT using np.nan analagously to datetimes

In [162]: y[1] = np.nan

In [163]: y
Out[163]: 
0              NaT
1              NaT
2   1 day, 0:00:00
Dtype: timedelta64[ns]

# works on lhs too
In [64]: s.max() - s
Out[64]: 
0    2 days, 0:00:00
1     1 day, 0:00:00
2            0:00:00
Dtype: timedelta64[ns]

In [65]: datetime(2011,1,1,3,5) - s
Out[65]: 
0    -365 days, 3:05:00
1    -366 days, 3:05:00
2    -367 days, 3:05:00
Dtype: timedelta64[ns]

In [66]: timedelta(minutes=5) + s
Out[66]: 
0   2012-01-01 00:05:00
1   2012-01-02 00:05:00
2   2012-01-03 00:05:00
Dtype: datetime64[ns]
```