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-docutils/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyDocutils(Package):
+class PyDocutils(PythonPackage):
     """Docutils is an open-source text processing system for processing
     plaintext documentation into useful formats, such as HTML, LaTeX,
     man-pages, open-document or XML. It includes reStructuredText, the
@@ -35,9 +35,6 @@ class PyDocutils(Package):
     homepage = "http://docutils.sourceforge.net/"
     url      = "https://pypi.python.org/packages/source/d/docutils/docutils-0.12.tar.gz"
 
-    version('0.12', '4622263b62c5c771c03502afa3157768')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))
+    version('0.13.1', 'ea4a893c633c788be9b8078b6b305d53',
+            url="https://pypi.python.org/packages/05/25/7b5484aca5d46915493f1fd4ecb63c38c333bd32aa9ad6e19da8d08895ae/docutils-0.13.1.tar.gz")
+    version('0.12',   '4622263b62c5c771c03502afa3157768')