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.
Some really bad things :) when the version number is provided instead of simply use `stable` or `dev`.

1. get all the release availables with `https://www.googleapis.com/storage/v1/b/dart-archive/o?prefix=channels/stable/release/&delimiter=/` . This will return something like:

  ```json
{
 "kind": "storage#objects",
 "prefixes": [
  "channels/stable/release/29803/",
  "channels/stable/release/30036/",
  "channels/stable/release/30107/",
  "channels/stable/release/30188/",
  "channels/stable/release/30798/",
  "channels/stable/release/31822/",
  "channels/stable/release/32314/",
  "channels/stable/release/33014/",
  "channels/stable/release/34825/",
  "channels/stable/release/35121/",
  "channels/stable/release/35530/",
  "channels/stable/release/36345/",
  "channels/stable/release/36647/",
  "channels/stable/release/37348/",
  "channels/stable/release/37644/",
  "channels/stable/release/37942/",
  "channels/stable/release/37972/",
  "channels/stable/release/38663/",
  "channels/stable/release/39553/",
  "channels/stable/release/41096/",
  "channels/stable/release/42013/",
  "channels/stable/release/42039/",
  "channels/stable/release/latest/"
 ]
}
  ```

2. For each `prefixes`:
  1. Get the VERSION file from something like `https://storage.googleapis.com/dart-archive/$prefix/VERSION`:

    ```json
{
  "revision": "42039",
  "version" : "1.8.3",
  "date"    : "201412011009"
}
    ```

    2. if the version matches the provided version. If true break the loop and download the version at `https://storage.googleapis.com/dart-archive/$prefix/sdk/dartsdk-linux-x64-release.zip` else continue with the next prefix.

Another loop has to be done with the Dev channel https://www.googleapis.com/storage/v1/b/dart-archive/o?prefix=channels/dev/release/&delimiter if not found.

### install Dart with the archive

Once the good build number has been found, it's quite easy:

```shell
curl http://storage.googleapis.com/dart-archive/$goodPrefix/sdk/dartsdk-linux-x64-release.zip > dartsdk-linux-x64-release.zip
unzip dartsdk-linux-x64-release.zip > /dev/null
rm dartsdk-linux-x64-release.zip
export DART_SDK="$PWD/dart-sdk"
export PATH="$DART_SDK/bin:$PATH"
```

NB: for `stable` I can use `channels/stable/release/latest` as prefix and for `dev`I can use `channels/dev/release/latest` as prefix and get rid of the process describe above.

I hope that it's clear :)