feat(core): make valdrClasses configurable via valdr service

- add setClasses function to valdr
- use bootstraps default classes 'has-success' and 'has-error' as the default valid/invalid classes

closes #6
chore(valdrEvents): rename event used to trigger a re-validation
Merge pull request #10 from philippd/master

feat(core): make valdrClasses configurable via valdr service