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-functools32/package.py

@@ -25,17 +25,11 @@
 from spack import *
 
 
-class PyFunctools32(Package):
+class PyFunctools32(PythonPackage):
     """Backport of the functools module from Python 3.2.3 for use on 2.7 and
     PyPy."""
 
     homepage = "https://github.com/MiCHiLU/python-functools32"
-    # base https://pypi.python.org/pypi/functools32
     url      = "https://pypi.python.org/packages/source/f/functools32/functools32-3.2.3-2.tar.gz"
 
     version('3.2.3-2', '09f24ffd9af9f6cd0f63cb9f4e23d4b2')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))