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-cclib/package.py

@@ -25,18 +25,13 @@
 from spack import *
 
 
-class PyCclib(Package):
+class PyCclib(PythonPackage):
     """Open source library for parsing and interpreting the results of
     computational chemistry packages"""
 
     homepage = "https://cclib.github.io/"
-    url      = "https://github.com/cclib/cclib/releases/download/v1.5/cclib-1.5.tar.gz"
 
-    version('1.5', 'c06940101c4796bce82036b13fecb73c')
-
-    extends('python')
+    version('1.5.post1', '1a50be48e4597b1a6dabe943da82a43c',
+            url="https://github.com/cclib/cclib/releases/download/v1.5/cclib-1.5.post1.tar.gz")
 
     depends_on('py-numpy@1.5:', type=('build', 'run'))
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))