Added UMD (AMD/CommonJS(Browserify) support). Fixes #21

It can now be used in Browserify with:
```js
var twitterFetcher = require('twitter-fetcher');
```

And require.js with:
```js
require([‘twitter-fetcher’], function (twitterFetcher) { … })
```

Normal usage is still supported (your example file works)

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I avoided adjusting `twitterFetcherCallback`'s indentation for now, so you can clearly see that code change is minimal. If interested I'll fix that and add those examples to the readme.