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-pil/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyPil(Package):
+class PyPil(PythonPackage):
     """The Python Imaging Library (PIL) adds image processing capabilities
     to your Python interpreter. This library supports many file formats,
     and provides powerful image processing and graphics capabilities."""
@@ -39,8 +39,4 @@ class PyPil(Package):
 
     # py-pil currently only works with Python2.
     # If you are using Python 3, try using py-pillow instead.
-    extends('python')
     depends_on('python@1.5.2:2.8')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)