implemented flexible callback style for groupBy and countBy
added winner to perf race output (see #11)
temporarily getting rid of the 1000-element tests

also getting rid of the underscore tests...
because seriously, who cares about underscore?
refactored perf_race.js to speed things up
switched from 1000- to 5-element array perf test case
added the option to specify only a specific race to run (see #11)
fixed up comparer used for perf races (see #11)
added races for sortBy, groupBy, and countBy
added uniq race (see #11)
added zip race (see #11)
ugh, dumb name collision
more refactoring!
added races for shuffle and flatten
added races for difference, union, and intersection (see #11)
capitalize those headers
added value selector support to min and max
hmm... lazy is getting a bit whipped here :(
updated sum to accept a value selector
refactored perf_race.js to leverage race.js more
fixed min and max to be consistent w/ underscore and lo-dash

Oddly (to me), these libraries return -Infinity for max([]) and Infinity
for min([])... weird, but great for performance!