Re-order documentation for astropy.units

I've come across several users who have read the start of the docs for astropy.units and use the original API which involved finding the conversion factor with pure units, then multiplying numbers, rather than using Quantities. I think that we should consider re-ordering the 'getting started' section to start straight with quantities, and then show the pure unit stuff later since it's not as useful now we have Quantity.

@mdboom - what do you think?