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-ptyprocess/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyPtyprocess(Package):
+class PyPtyprocess(PythonPackage):
     """Run a subprocess in a pseudo terminal"""
 
     homepage = "https://pypi.python.org/pypi/ptyprocess"
@@ -33,9 +33,4 @@ class PyPtyprocess(Package):
 
     version('0.5.1', '94e537122914cc9ec9c1eadcd36e73a1')
 
-    extends('python')
-
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))