underscore.js 一个强大的js函数库
Underscore提供的100多个函数，主要涉及对Collection、Object、Array、Function的操作：
Collections（集合）
each, map, reduce, reduceRight, find, filter, reject, all, any, include, invoke,pluck, max, min, sortBy, groupBy, sortedIndex, shuffle, toArray, size
Arrays（数组）
first, initial, last, rest, compact, flatten, without, union, intersection,difference, uniq, zip, indexOf, lastIndexOf, range
Functions（函数）
bind, bindAll, memoize, delay, defer, throttle, debounce, once, after, wrap,compose
Objects（对象）
keys, values, functions, extend, defaults, clone, tap, isEqual, isEmpty,isElement, isArray, isArguments, isFunction, isString, isNumber, isBoolean,isDate, isRegExp, isNaN, isNull, isUndefined
Utility（功能）
noConflict, identity, times, mixin, uniqueId, escape, template
