upgrading to lodash 4 and only requiring individual functions.

[ekryski]

Closes #17.

[daffl]

I don't think this is how it works unless Babel added Tree shaking. We'll probably have to do

import isEmpty from 'lodash/isempty';
import extend from 'lodash/extend';

etc. Or to make it much easier and not having to change the rest of the code:

const _ = {
  extend: require('lodash/extend'),
  isEmpty: require('lodash/is-empty')
}

[daffl]

I'm going to close this one in favour of #19