Add default import in twitter.js

[kissarat]

There declaration in index.d.ts

export default class Twitter {

But without this PR it is impossible to import this way

import Twitter from 'twitter-lite'

Because for exporting

module.exports = Twitter

Import must looks like

import * as Twitter from 'twitter-lite'

But it's not described in index.d.ts

[fdebijl]

Shouldn't this line be moved above module.exports, or is it hoisted?

[kissarat]

Maybe such code are more clear, but it's not possible to put module.exports.default = Twitter before module.exports = Twitter

[kissarat]

@fdebijl I'm not sure that I'm not sure that I understand you well, but I made an appropriate changes