Add PythonPackage base class

- Add a PythonPackage class with build system support.
  - Support build phases in PythonPackage
  - Add a custom sanity check for PythonPackages
  - Get rid of nolink dependencies in python packages

- Update spack create to use new PythonPackage class

- Port most of Python packages to new PythonPackage class

- Conducted a massive install and activate of Python packages.
  - Fixed bugs introduced by install and activate.

- Update API docs on PythonPackage

var/spack/repos/builtin/packages/py-unittest2py3k/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyUnittest2py3k(Package):
+class PyUnittest2py3k(PythonPackage):
     """unittest2 is a backport of the new features added to the unittest
     testing framework in Python 2.7 and 3.2. This is a Python 3 compatible
     version of unittest2."""
@@ -35,8 +35,5 @@ class PyUnittest2py3k(Package):
 
     version('0.5.1', '8824ff92044310d9365f90d892bf0f09')
 
-    extends('python')
+    depends_on('python@3:')
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)