Add equivalencies to UnitRegistry, as well as add/set/context managers
Add documentation for the equivalency context manager
Use equivalencies=None, so that empty list can override global default
Cleanup for equivalencies=None; further tests
Adapt doctest to deal with small representation errors
Add comment to decompose to ensure no equivalencies are ever passed
Reinterpret equivalencies=[] and None; normalize on UnitRegistry input
Separate out context manager to bottom; add in-text references, cleanup
Spelling corrections in docs
Correct external references in documentation
Additional minor changes after rebase (corrections to quantity.py/CHANGES.rst)