Added validation

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

I was scratching my head for a while when I uploaded files using the test application - I was getting a `File '' not found` message. Turns out the file exceeded my max_filesize.

This PR implements the validate method in UploadFileHelper. I also prepared a symfony PR as I think it would be a better place to have this in some way (currently the various UPLOAD_ERR_* constants are checked only in the Validation component).

supersedes https://github.com/symfony-cmf/MediaBundle/pull/55