Implemented like-operator operations for operator proxies
Added Operator#abs2 to help with pretending to be a Numeric
Fixed handling of nil-units in respond_to_missing?
Added unary operators to the operator proxies
Prevent Operators from being initialized without operands
Implemented cross-operator operations for operator proxies
Optimized operator proxies involving zero

Anything for which zero? returns true is considered to be zero. This doesn't check for divide-by-zero because, ideally, that should already be handled by the ruby core.
Moved common addition and subtraction methods to Operator
Added the SquareRoot operator

Updated the Math.sqrt override to handle Operators and wrap them in a SquareRoot proxy
Collapse compatible operands when adding or subtracting operator proxies
More optimizations for operator proxy operations that should return 0
Fixed the divide method in UnitsMixin

It was causing a coercion-loop when trying to divide a Rational by a Units::Operator
More optimizing
Added some testing and minor fixes for Rational

Rational is still a mess, but this helps a little
Short-circuit Operator#eql? when other is 0
Fixed handling of conversions for wrappers without units

If a Units::Numeric wrapper was initialized without units, try to assign units using a conversion operator would cause a crash when calling @units.valid_conversion?. The new behavior simply assigns the desired units. This may not be ideal, but it fixes the crash.
Better addition and subtraction of zero

When adding, or subtracting, a 0-value to an Addition or Subtraction operator, and the operator has only one element, return that one element instead of a new operator. This is a small optimization that reduces the buildup of the huge operator trees that are often the result of complex operations.
Prevent reduce() from mixing units and non-units

Converting bare literals contained in Operators should be saved until the Operator is converted to its final units, otherwise it's hard to know which units to convert it to.
Readme fixup for querying units
Added per_<unit> conversions

Now 3.meters.per_second will do the right thing