[Validator] Add digit constraint

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | 

This PR adds a constraint for validating that a value consists of numeric characters only. Also added german translation.

This is my first contribution to the Symfony2 repository. I hope I have not forgotten anything, otherwise please tell me =)