Enable viewing of (most) notebookes with Python 3.x

Just fixed a few print statements so that notebooks run with Python 3.x.

The Quantities tutorial still fails, however, due to an issue with scalar conversion:

```
ERROR:astropy:TypeError: Only dimensionless scalar quantities can be converted to Python scalars
ERROR: TypeError: Only dimensionless scalar quantities can be converted to Python scalars [astropy.units.quantity]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-12-7694f196ce41> in <module>()
----> 1 sigma_scalar = math.sqrt(np.sum((v - np.mean(v))**2) / np.size(v))

/usr/lib/python3.4/site-packages/astropy/units/quantity.py in __float__(self)
    719     def __float__(self):
    720         if not self.isscalar or not self.unit.is_unity():
--> 721             raise TypeError('Only dimensionless scalar quantities can be '
    722                             'converted to Python scalars')
    723         else:

TypeError: Only dimensionless scalar quantities can be converted to Python scalars
```