feat(message): first version of valdr message output

- uses either simple strings from the constraint or message key with angular-translate
- allows customizable message templateUrl or template

see #5
chore(message): add some documentation and clean up code
test(valdrMessage): add test for valdrMessage service
test(message): add tests for vladrMessage service and directive
chore(demo): updated demo
Merge pull request #12 from philippd/master

feat(message): add message module