added reduce methods to NSArray and NSSet

Return a single value from an array or set by iterating through the
elements and transforming a running total.
!!BREAKING - changed -map to put NSNulls instead of ignoring objects
"Merge pull request #75 from robinsenior/reduce\n\nadded reduce methods to NSArray and NSSet"
add -reduce counterpart without initial value
make NSSet reduce tests stable
Merge pull request #81 from supermarin/reduce

Reduce