Import Kept code !

If Kept ought to be our reference app, its integration with Daybed should be smooth, consistent and dead simple!

We can't pretend to be sexy and expect developpers to write complex code like in https://github.com/n1k0/kept/pull/26

That's why I took the code from Kept daybed storage and propose to include it in daybed.js. If we needed this for Kept, others will for their app.

Only thing I decided to leave in Kept (and we might discuss it), is the concatenation of records from different models into a single list...

Daybed storage in Kept becomes : 

```javascript
DaybedStore.prototype = {
  setUp: function() {
    var self = this;
    return Daybed.startSession(self._options.host, {
      token: self._options.token
    })
    .then(function(session) {
      self._session = session;
      self._session.prefix = PREFIX + session.token.slice(0, 5);
      self._options.onSession(session);

      return self._session.addModels(self._types);
    });
  },

  load: function() {
    return this._session.getRecords(this._types.keys())
      .then(function (allRecords) {
        var data = [];
        for(var model in allRecords) {
          var records = allRecords[model];
          records.forEach(function (record) {
            record.type = model;
            data.push(record);
          })
        }
        return data;
      });
  },

  save: function(data) {
    var byModelId = {};
    data.forEach(function (record) {
      if (byModelId[record.type] === undefined)
        byModelId[record.type] = [];
      byModelId[record.type].push(record);
    });
    return this._session.synchronizeRecords(byModelId);
  }
};
```

* [x] Import code
* [x] Object equals util
* [x] Unit tests
* [ ] Decide if we import the pattern of storing ``type`` attribute in records to distinguish them (will make Kept integration a lot shorter)