Add `to` method to table columns

This PR adds a new meth to table columns called `to`.  It's analogous to the `to` of `Quantity`, in that it returns a `Quantity` object in the requested units.

It also works as an "easy" way to get out `Quantity` objects if you do ``.to(None)``.  I say "easy" because it's really just syntactic sugar for u.Quantity(table['colname'])... But I find the latter to be more awkward to use in regular code, so I like the option to call a method that yields a Quantity.  And the unit-conversion is of course valuable in various circumstances.

cc @taldcroft @astrofrog @mhvk
Actually I think that we want to be careful here - because users might then assume that these always behave as quantities. This would be a good place to add a caveat regarding this. In fact, I think we should actually not recommend the style below but instead recommend things like ``t['a'].quantity + .005 * u.km``. Adding the ``quantity`` attribute makes it safer and will make things like ``np.sin(t['a'].quantity)`` work as expected.