Collection of validation errors

We added the ability to collect the validation errors in order to display them at once (e.g. in a form).

Usage: 
```ruby
collect_validation_errors true

rescue_from Grape::Exceptions::Validations do |e|
  Rack::Response.new({
    'status' => 422,
    'message' => e.message,
    'errors' => e.errors
  }.to_json, 422)
end
```

We made sure that the exception handling is backward compatible. 

What do you think about it?