Aggregate multiple FormValidations into one

With this change `doCheckXxx` methods can return several indications of different `Kind`s.

Use case: https://github.com/jenkinsci/matrix-project-plugin/pull/10

![aggregated_validations](https://cloud.githubusercontent.com/assets/206841/4950314/837f1736-6659-11e4-96f7-17fbc5a39a80.png)