Added feature: package extensions

- packages can be "extended" by others
- allows extension to be symlinked into extendee's prefix.
- used for python modules.
  - first module: py-setuptools

var/spack/packages/python/package.py

@@ -1,10 +1,13 @@
 from spack import *
 
+
 class Python(Package):
     """The Python programming language."""
     homepage = "http://www.python.org"
     url      = "http://www.python.org/ftp/python/2.7.8/Python-2.7.8.tar.xz"
 
+    extendable = True
+
     version('2.7.8', 'd235bdfa75b8396942e360a70487ee00')
 
     depends_on("openssl")