callable Payjp object

[wozozo]

Implemented a constructor Payjp that can be both function-called and constructor-called.

// es6 import statement
import Payjp from 'payjp';
var payjp1 = new Payjp('apikey');

// require
var _payjp = require('payjp');
var payjp2 = new _payjp('apikey');

// callable
var payjp3 = require('payjp')('apikey');