Add authorized_to_contribute to users show API.

:fork_and_knife: 

Adds the `authorized_to_contribute` boolean to the users show API.

The blob now looks like:

``` json
{
  "username": "brettchalupa",
  "name": "Brett Chalupa",
  "company": "FullStack",
  "github": [
    "brettchalupa"
  ],
  "twitter": "brettchalupa",
  "irc": "",
  "jira": "",
  "authorized_to_contribute": true,
  "cookbooks": {
    "owns": {
      "redis": "http://localhost:3000/api/v1/cookbooks/redis"
    },
    "collaborates": {},
    "follows": {
      "haskell": "http://localhost:3000/api/v1/cookbooks/haskell",
      "mysql": "http://localhost:3000/api/v1/cookbooks/mysql",
      "yum": "http://localhost:3000/api/v1/cookbooks/yum"
    }
  }
}
```