Adds root and exclude to oauth2

please merge #603 first


After merging this pull request the oauth2 middelware can be user like this:
```ruby
use Grape::Middleware::Auth::OAuth2, token_class: 'FakeToken', root: '/api', exclude: %w(version ping)
```

This means that the filter will apply only to urls under `/api` and it will exclude `version` and `ping` so you can call `/api/v1/version` without warring about access_token.

The `require` option is not powerful enough to cover all scenarios.