Add instant answer for packagist composer package search

**What does your Instant Answer do?**
Returns package information from packagist composer package manager's registry.

**What problem does your Instant Answer solve (Why is it better than organic links)?**
It's is easy to go to the package's page directly instead going to packagist website and searching for the package.

**What is the data source for your Instant Answer? (Provide a link if possible)**
http://packagist.org

**Why did you choose this data source?**
This is the only source which lists composer packages.

**Are there any other alternative (better) data sources?**
No.

**What are some example queries that trigger this Instant Answer?**
composer laravel, packagist laravel, composer install laravel

**Which communities will this Instant Answer be especially useful for? (gamers, book lovers, etc)**
PHP Programmers.

**Is this Instant Answer connected to a DuckDuckHack [Instant Answer idea](https://duck.co/ideas)?**
No.

**Which existing Instant Answers will this one supersede/overlap with?**
Nothing.

**Are you having any problems? Do you need our help with anything?**
No.

**What are the terms of use for the API? Will DuckDuckGo need specific authorization (e.g. an API key)? Are there any costs associated with API usage?**
It is a public API.

**Where did you hear about DuckDuckHack? (For first time contributors)**
From FastCo Labs article.

**What does the Instant Answer look like? (Provide a screenshot for new or updated Instant Answers)**
![ddg-packagist](https://cloud.githubusercontent.com/assets/873785/5207478/a3c0d286-7578-11e4-90df-3e334d603586.png)

## Checklist
Please place an 'X' where appropriate.

```
[x] Added metadata and attribution information
[x] Wrote test file and added to t/ directory
[x] Verified that instant answer adheres to design guidelines (https://duck.co/duckduckhack/design_styleguide)
[x] Verified that instant answer adheres to code styleguide (https://duck.co/duckduckhack/code_styleguide)
[] Tested cross-browser compatibility

    Please let us know which browsers/devices you've tested on:
    - Windows 8
        [] Google Chrome
        [] Firefox
        [] Opera
        [] IE 10

    - Windows 7
        [] Google Chrome
        [] Firefox
        [] Opera
        [] IE 8
        [] IE 9
        [] IE 10

    - Windows XP
        [] IE 7
        [] IE 8

    - Mac OSX
        [x] Google Chrome
        [] Firefox
        [] Opera
        [] Safari

    - iOS (iPhone)
        [] Safari Mobile
        [] Google Chrome
        [] Opera

    - iOS (iPad)
        [] Safari Mobile
        [] Google Chrome
        [] Opera

    - Android
        [] Firefox
        [] Native Browser
        [] Google Chrome
        [] Opera
```
The source URL is what get's used by the "More at" link, for users to see more info. We should probably send them to the normal Packagist site search rather than the JSON API results