Dart support

This will add support for [Dart](https://www.dartlang.org/).

```yaml
language: dart
```

You can provide the version to use with the `dart` property. `stable` is the default and you can use either `dev` or a version number.

If the project contains a `pubspec.yaml` file then `pub get` is launch.

Finally the script run `dart test/all_test.dart` if `test/all_test.dart` exists.
Acutally the process I described was to handle any Dart version. If we end up to support only the latest version from `stable` and `dev` channels it will be a really simple script. Should we support older version ? What do you think ?

I will update the PR to avoid `sudo` and to only support `dev` and `stable`.