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
that was intended to indicate that the ``style`` keyword only applies for scalars (which I found quite weird about ``array2string``) - but looking at it again I see that didn't really get the message across.  Anyone fiddling with this will probably have to read the array2string docs anyway, so I just got rid of it as you suggest.