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
```
"composer" is likely to broad of a trigger. E.g. "music composer" would trigger this and return results which likely aren't relevant to this query. Let's change it to "composer packages" or "php composer" etc -- triggers that better indicate the users intentions