Merge pull request #2061 from mdboom/wcs/missing-c-extension

Display a better error message if the _wcs C extension can not load.
Correct problems with UT1->UTC->UT1 roundtrip; add regression tests
Clarify CHANGES.rst
Merge pull request #2077 from mhvk/time-ut1-leapsec

UT1->UTC conversion wrong on leap seconds
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.