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-pmw/package.py

@@ -25,15 +25,10 @@
 from spack import *
 
 
-class PyPmw(Package):
+class PyPmw(PythonPackage):
     """Pmw is a toolkit for building high-level compound widgets, or
        megawidgets, constructed using other widgets as component parts."""
     homepage = "https://pypi.python.org/pypi/Pmw"
     url      = "https://pypi.python.org/packages/source/P/Pmw/Pmw-2.0.0.tar.gz"
 
     version('2.0.0', 'c7c3f26c4f5abaa99807edefee578fc0')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)