Issue Category Filter Feature

Overview
------------

![screenshot-categories](https://cloud.githubusercontent.com/assets/222509/4872679/bee35fb8-61ee-11e4-9205-1f5a616aa20c.png)

This feature adds a category-filter to the navigation bar that allows you to filter your issues.


Use Cases
---------------

* Categorize your issues by country, region, type (paid, free) or content- category (Fantasy, Science-Fiction, etc...)
* Combine multiple pseudo-magazines in a single app


Configuration
------------------

When categories are specified in the issue list (shelf.json manifest), a Category- Dropdown will be shown in the shelf navigation bar.

Categories can be set in the following way (shelf.json):
``` json
[
    {
      "name": "baker-framework-tutorial",
      "title": "Baker Framework Tutorial",
      "info": "New to Baker? Start with this tutorial.",
      "date": "2015-01-25 13:01:25",
      "cover": "http://bakerframework.com/demo/baker-framework-tutorial.png",
      "url": "http://bakerframework.com/demo/baker-framework-tutorial.hpub",
      "categories": ["Support"]
    },
  {
    "name": "a-study-in-scarlet",
    "title": "A Study in Scarlet",
    "info": "The original story by Sir Arthur Conan Doyle, published the first time in 1887.",
    "date": "2016-11-24 09:00:00",
    "cover": "http://bakerframework.com/demo/a-study-in-scarlet.png",
    "url": "http://bakerframework.com/demo/a-study-in-scarlet.hpub",
    "categories": ["Fantasy"]
  }
]
```

If, in Localizable.strings, you set "ALL_CATEGORIES_TITLE" to an empty string (""), it will not show the "All Categories" item, and instead use the first category by default.

The application will automatically scan all set categories and show a (sorted) list in the dropdown. If no categories are set at all, the dropdown will not be visible (this allows backwards compatibility).


Bugs / Caveats
--------------------

When an external keyboard is attached (in Simulator: Hardware > Keyboard > Connect Hardware Keyboard), the list of categories will not show when pressing the Category List button in the navigation bar.