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/scons/package.py

@@ -25,14 +25,9 @@
 from spack import *
 
 
-class Scons(Package):
+class Scons(PythonPackage):
     """SCons is a software construction tool"""
     homepage = "http://scons.org"
     url      = "http://downloads.sourceforge.net/project/scons/scons/2.5.0/scons-2.5.0.tar.gz"
 
     version('2.5.0', '9e00fa0df8f5ca5c5f5975b40e0ed354')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)