Added composer package support

Hi! I thought it'd be prudent to add composer package support for this so that it can be easily included in a composer-based app. Right now, the way I am including this is:

**composer.json:**
```
{
  "repositories": [
    {
      "type": "vcs",
      "url": "//github.com/Xorcode/phalcon-oauth2"
    }
  ],
  "require": {
    "sum/phalcon-oauth2": "dev-master"
  }
}
```

I'm sure you'll want to change some things, update the documentation, and so forth.