Add composer.json

This PR adds a composer.json file, which would allow AffiliateWP to be installed via [Composer](https://getcomposer.org/), the PHP dependency management tool. I borrowed the format from the [composer.json on Yoast's WordPress SEO plugin](https://github.com/Yoast/wordpress-seo/blob/master/composer.json).

A sample composer.json that adds WordPress core (trunk) and AffiliateWP (the latest tagged version) as dependencies might look like this:

```json
{
	"name": "chrisvanpatten/sample-project",
	"description": "My Sample Project is great.",
	"type": "project",
	"repositories": [
		{
			"type": "vcs",
			"url" : "https://github.com/affiliatewp/AffiliateWP",
			"no-api": "true"
		}
	],
	"require": {
		"johnpbloch/wordpress": "dev-master",
		"affiliatewp/AffiliateWP": "*"
	},
	"extra": {
		"wordpress-install-dir": "wp",
		"installer-paths": {
			"content/plugins/{$name}":    ["type:wordpress-plugin"],
			"content/mu-plugins/{$name}": ["type:wordpress-muplugin"],
			"content/themes/{$name}":     ["type:wordpress-theme"]
		}
	}
}
```

Add this to your project root, install Composer, and run `composer install` and it should automatically install WordPress and your plugins to the paths specified in the "extra" block.