Fix function signatures in documentation
Merge pull request #40 from mwcraig/logging

Initial implementation of logging

Now that function signatures are preserved in the docs this is ready, I think....though I fully expect it to change down the road.
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