Add methods to manage deployment keys

Added methods to manage gitlab deployment keys. Examples:

```ruby
api.deploy_keys("project_id")
api.create_deploy_key("project_id", "key title", "key contents")
api.delete_deploy_key("project_id", "key_id")
```

Most of the functionality works fine,  except `create_deploy_key`. 
Gitlab has a bug with key creation which triggers into 404 error, 
once its fixed everything should be good.

See bug details here: https://github.com/gitlabhq/gitlabhq/issues/4241
Its closed, but the issue still persists.