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.
It seems to be possible (with more work) but it would perhaps be less reliable as I have to do something similar to what is done in [Dart Download Archive](https://www.dartlang.org/tools/download-archive/) to display the available versions. There is not a simple url template to get the archive. The archives are identified by a build number and you have to go into each folder and look at `VERSION` to get the version of the build.

I will try to update my PR but as I am not really fluent in shell that could be really buggy.