added Msun alias for solar mass unit

This is a very simple change that just adds ``"Msun"`` as an alias for solar mass, in addition to ``"Msun"``.  

The main motivation for this is that I see a lot of ASCII tables that get sent around that have "Msun" instead of "M_sun", and this makes it a bit easier to parse such things.  

@mdboom, what do you think?