Default serialize implementation (Fix #358)

[SBoudrias]

To continue the discussion over a suggested implementation and allow people to test it out!

On a side note, I got different behavior in Underscore between Node and the browser. In the browser, undefined template value were ignored as in node they throwed error.

[nicoburns]

Just thought I would contribute our default serialize function:

serialize: function() {
    if (this.model) {
        return this.model.toJSON();
    }
    else if(this.collection) {
        return {
            items: this.collection.toJSON(),
            hasItems: this.collection.length > 0
        };
    }
    else {
        return {};
    }
},

We've been using a version of this for about 6 months now, and it seems to cover most use cases where a custom serialize function isn't needed.

[abritinthebay]

The Node errors would be expected. It's a lot stricter.

[tbranyen]

@SBoudrias can you rebase this pull request to master? I think we should merge as-is.

[SBoudrias]

There it is!

[tbranyen]

Hey @SBoudrias feel free to merge once the build passes.