BUG: Fix and test unitless UnitArray raised to a power.
DOC: Record in CHANGES.
Merge branch 'master' into fix/unitless-power

* master:
  BUG: Fix doctests again.
  Document CHANGES.
  ENH: Document CHANGES.
  ENH: Make the parser handle offsets.
  ENH: Special-case all weird labels so all units can be round-tripped through the parser.
  BUG: Add S for Siemens.
  ENH: Remove trivial offsets from string representations.
  BUG: numpy changed how vectorize() keeps the original function.
  BUG: Clean up unit parser context.

Conflicts:
	CHANGES.txt
Merge pull request #28 from enthought/fix/unitless-power

BUG: Fix and test unitless UnitArray raised to a power.