CustomKernel fixes
Added CustomKernel tests for zero-summed kernels.
renamed deriv method to fit_deriv
rename 'deriv' to 'fit_deriv' in documenation
Clean up `setup.py test --help`
added an entry in the changelog
Make UTC the default time scale
Update tests for UTC default
Update docs for UTC default
Update CHANGES for UTC default
Fix spacing error
Fix rounding errors with fractional powers.
Simplify use of validate_power in Quantity.__array_prepare__ as
recommended by @mhvk
Add example about fractional powers.
Add CHANGES entry.
Convert floats into Fractions if they are close enough.
Disallow tuples as powers on Quantities
Fix test to test implicit float -> Fraction conversion.
Add more information in the documentation.
Merge pull request #2088 from mdboom/testing/commandline-help

Clean up `setup.py test --help`