Override basic arithmetic operations to allow operand to be scalar or Quantity or CCDData

Closes #38
Work around issue with multiplying units when one is dimensionless

The issue is reported at astropy/astropy#2377
Merge pull request #44 from mwcraig/mul-etc-take-2

Override basic arithmetic operators
Change linear gain to use automatic error propagation and better handle units
Add bounds checking to FITS slices

Closes #41
Merge pull request #46 from mwcraig/slice-check

And some sanity-checking to FITS slices and use error propagation for linear gain.