Pageable Collection for Backbone

Inspired by backbone-pageable, simple pagination replacement for Backbone.Collection.

Nothing too special, i'd recommend checking source code for more details, but all in all, this is just a
normal Collection with extra functions that you can use to fetch required paginated data from the server and
render simple pagination links whenever you want.

Comes with a reasonable defaults which would allow you to use Pageable Collection from the get go with
only several lines of extra backend code.

How to use

1\. Add link after backbone:

```javascript

``

2\. Back end setup:

Pageable Collection accepts 3 types of responses from the server.
a) JSON response with state and items.

``js
{
state: {
total: 30 // Set total available items.
},
items: [] // Array of models.
}
``

b) JSON response with models. Suited towards bassic pagination, when you provide
perPage + 1 items where extra item is an indicator that next page is available.

``js
[{id: 1, title: 'Foo'}, {id: 2, title: 'Bar'}] // Array of models.
``

c) JSON response with models + HTTP headers with state data.

``js
[{id: 1, title: 'Foo'}, {id: 2, title: 'Bar'}] // Array of models.
``
Available Headers are: X-Total, X-PerPage, X-CurrentPage.

3\. Use instead of default Backbone.Collection:

``javascript
var UsersCollection = Backbone.PageableCollection.extend({
url: 'http://example.com/users'
});
var users = new UsersCollection;
// Get first users page.
users.fetchPage(1, {reset: true});
users.on('reset', this.render, this);

// In render method
render: function(collection) {
// Loop through all of the models in collection.
// This is fine only for server mode where backed returns state
// with total items. Or if you're ok with perPage + 1 items on basic pagination mode.
collection.each(function(model) {

});
// Get only items at current page. Recommended for client mode and
// basic server pagination mode.
collection.getItems().each(function(item) {

});
// Render paginator. links methood picks paginator automatically based on state.
this.$el.append(collection.links().el);
}
```

That's it for now. I'll be adding better readme in the future, for now read code.