Added joinWrapper and moved to Webpack

Join Wrapper

The join wrapper handles missing input values. The first two parameters (the left array and the left accessor) are required. An error will be thrown if they are missing. The right array and right accessor are now optional. This allows one to define self-joins fairly easily.

_.hashLeftOuterJoin([{id: 'a'}], function (d) { return d['id']; });

or

_([{id: 'a'}]).hashLeftOuterJoin(function (d) { return d['id']; }).value();

Switched to Webpack

Webpack offers quite a few advantages of Browserify. Just to mention one, the amount of dev dependencies this library requires was reduced by 1/3.