Exposing Project Labels in the REST API

The intent here is to expose the tag_list property of the Project entity over the REST API so that any project searches include the information. The specific reason I've implemented this change is for an environment in which multiple gitlab servers exist, where a central portal to the projects that are spread around the network will be useful. Having access to filtering on this fairly large project list based on their labels, will be of great use.

This satisfies the feature request http://feedback.gitlab.com/forums/176466-general/suggestions/6325819-project-labels-via-api

The change was made in the `lib/api/entities.rb` file.

The output of a `GET` to something like `/projects` or `/projects/7` is now:
```javascript
{
  "id": 7,
  "description": "Veritatis est eaque voluptas magni expedita.",
  "default_branch": "master",
  **"tag_list": [
    "typeahead",
    "twitter"
  ],**
  "public": false,
  "archived": false,
  "visibility_level": 0,
  ...
}
```

See merge request !329