[fix] remove hoodie.generateId() from public API

BREAKING CHANGE: hoodie.generateId() is no more accessible.
We plan to expose it to plugin developers via

```js
Hoodie.extend(function(hoodie, lib, utils){
  var id = utils.generateId()
});
```

* * *

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Merge pull request #236 from hoodiehq/236_remove_generate_id_from_public_api

remove hoodie.generateId() from public API