Add support for latexing array quantities

See #2258 for context - this makes it so that array quantities produce decent-looking latex output for ipython notebook consumption.

Look good to you in concept, @mdboom and @mhvk?  If so, I can add tests to check this properly
OK, I definitely did *not* get that. Note that you could replace `np.array(self.value, copy=False)` with `self.view(np.ndarray)` (since `Quantity` scalars are stored as array scalars).