Testable install script

* wraps main code into a function
* EnvVar-based install start
* Clean environment after install

package.json

@@ -10,7 +10,8 @@
     "test/fast": "urchin -f test/fast",
     "test/slow": "urchin -f test/slow",
     "test/installation": "urchin -f test/installation",
-    "test/sourcing": "urchin -f test/sourcing"
+    "test/sourcing": "urchin -f test/sourcing",
+    "test/install_script": "urchin -f test/install_script"
   },
   "repository": {
     "type": "git",