Some more isType functions improvements

[FarSeeing]

Although you accepted my pull request not long ago, I am still not happy about some isType functions 😃

There are several variants, depending on the goal of the library itself and I'd like to see your guide for this.

1. to minify code:

var objectTypes = ['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp'];
if (!nativeIsArray)
{
      objectTypes.push('Array');
}
each(objectTypes , function(name) {
      _['is' + name] = function(obj) {
            return toString.call(obj) == '[object ' + name + ']';
      };
});

Maybe some more minifications.

1. to maximize performance:
   move isFunction, isRegExp, isDate functions to their own declarations using typeof/instanceof:

_.isFunction = function(obj) {
      return typeof obj === 'function';
}
_.isRegExp = function(obj) {
      return obj instanceof RegExp;
}
_.isDate = function(obj) {
      return obj instanceof Date;
}

see http://jsperf.com/isfunction-performance and http://jsperf.com/isregexp-test-for-regexp for performance graphs

1. to unify code branches:

each(['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp', 'Null', 'Undefined'], function(name) {
      _['is' + name] = function(obj) {
            return toString.call(obj) == '[object ' + name + ']';
      };
});
if (!_.isNull(null)) {
      _.isNull = function(obj) {
            return obj === null;
      };
}
if (!_.isUndefined()) {
      _.isUndefined = function(obj) {
            return obj === void 0;
      };
}

[jashkenas]

The #2 plan in this list won't fly -- because those tests won't work on objects from different frames ... but otherwise, feel free to send a pull request if you think you have an improvement on the current implementation.