Assorted changes

[mweastwood]

No real new functionality here. Just a slew of changes I've had built up.

• Strip measurement set related functionality out into a separate package (so that I can use it elsewhere)
• Add support for the Sun as a source (other solar system bodies that are recognized by CasaCore can now be added trivially)
• Add a command line tool for peeling
• Delete the diagnosis tool (because I don't want to support it)
• Rename bandpass to gaincal
• Revert polcal to use (Mitch|Stef)Cal iterations again
• Minor additions to the README