Add yo as peerDependency

By adding `yo` as peer dependency, the user only has to `npm install -g generator-canjs` and is good to go.

I also added `npm >= 1.2.10` as engine requirement which shows a non-fatal warning in case the user is using an earlier version of npm, which doesn't support peer dependencies yet.

/ref yeoman/generator#305

package.json

@@ -4,7 +4,7 @@
   "description": "CanJS generator for Yeoman",
   "keywords": [
     "yeoman-generator",
-    "canjs", 
+    "canjs",
     "canjs-generator"
   ],
   "homepage": "https://github.com/retro/generator-canjs",
@@ -29,8 +29,12 @@
   "devDependencies": {
     "mocha": "~1.9.0"
   },
+  "peerDependencies": {
+    "yo": ">=1.0.0-rc.1.4"
+  },
   "engines": {
-    "node": ">=0.8.0"
+    "node": ">=0.8.0",
+    "npm": ">=1.2.10"
   },
   "licenses": [
     {