Roll Observatory packages and add a roll script

packages/which/README.md

+which [![pub package](http://img.shields.io/pub/v/which.svg)](https://pub.dartlang.org/packages/which) [![Build Status](https://drone.io/github.com/seaneagan/which.dart/status.png)](https://drone.io/github.com/seaneagan/which.dart/latest) [![Coverage Status](https://img.shields.io/coveralls/seaneagan/which.dart.svg)](https://coveralls.io/r/seaneagan/which.dart?branch=master)
+=====
+
+Check for and locate installed executables.  Just like unix [which(1)][unix_which], except:
+
+* Doesn't shell out (fast).
+* Cross-platform (works on windows).  
+
+## Install
+
+```shell
+pub global activate den
+den install which
+```
+
+## Usage
+
+```dart
+import 'dart:io';
+
+import 'package:which/which.dart';
+
+main(arguments) async {
+
+  // Asynchronously
+  var git = await which('git', orElse: () => null);
+
+  // Or synchronously
+  var git = whichSync('git', orElse: () => null);
+
+  if (git == null) {
+    print('Please install git and try again');
+    exit(1);
+  }
+
+  await Process.run(git, ['add', '-A']);
+  await Process.run(git, ['commit', '-m', arguments.first]);
+}
+```
+
+[unix_which]: http://en.wikipedia.org/wiki/Which_%28Unix%29