Map/Set should treat -0 and +0 as identical keys.

The latest spec (Jan 20, 2014) uses SameValueZero (not SameValue) for
comparisons.  This is a change in the spec.

Closes: #129
Merge pull request #199 from cscott/gh-129

Map/Set should treat -0 and +0 as identical keys.
Automatically rewritten for npm.