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.
@joshk: In the `ubuntu:12.04` image there is no `unzip` to extract `dartsdk-linux-x64-release.zip` and there's nothing simple to work around. Is it possible to add `unzip` in [the default environment](http://docs.travis-ci.com/user/ci-environment/#Environment-common-to-all-VM-images) ?

PS: The absence of `sudo` gives me headaches. :)