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-cython/package.py

@@ -25,21 +25,16 @@
 from spack import *
 
 
-class PyCython(Package):
+class PyCython(PythonPackage):
     """The Cython compiler for writing C extensions for the Python language."""
     homepage = "https://pypi.python.org/pypi/cython"
     url      = "https://pypi.io/packages/source/c/cython/Cython-0.25.2.tar.gz"
 
     version('0.25.2', '642c81285e1bb833b14ab3f439964086')
-    
+
     version('0.23.5', '66b62989a67c55af016c916da36e7514')
     version('0.23.4', '157df1f69bcec6b56fd97e0f2e057f6e')
 
     # These versions contain illegal Python3 code...
     version('0.22', '1ae25add4ef7b63ee9b4af697300d6b6')
     version('0.21.2', 'd21adb870c75680dc857cd05d41046a4')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)