Expose WordCloud on the global object to prevent errors with RequireJS

Hi,

this PR exposes WordCloud on the global object to get this awesome library working with the (latest) RequireJS version.

It simply exposes WordCloud before the define('wordcloud', [], function() { return WordCloud; }); call.

Notice: As it uses a named AMD module called wordcloud the RequireJS configuration should look like:

requirejs.config({
  baseUrl: 'bower_components',
  paths: {
    "wordcloud"            : "wordcloud2.js/src/wordcloud2",
  }
});

Then the WordCloud module is available. I did testings with:

and wordcloud2 in version 1.0.5 and master, Final output of grunt test is:

Took 2102ms to run 23 tests. 23 passed, 0 failed.

Done, without errors.

What do you think about merging this PR?

timdream / wordcloud2.js