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.
@sethladd : exactly you can put the version in the `.travis.yaml` file. You can even put several versions.

The simplest file that will use the latest stable version of Dart is:
 
```yaml
language: dart
```

A version to run against multiple versions:

```yaml
language: dart
dart:
  - stable
  - dev
  - 1.7.2
  - 1.6.0
```

My main problems are:

- to manage to get it work without `sudo`.
- the missing of a simple link to get the Dart version of a `x.y.z` version. I have to work around that (see https://github.com/travis-ci/travis-build/pull/357#discussion_r21815545).