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-pudb/package.py

@@ -26,7 +26,7 @@
 from spack import *
 
 
-class PyPudb(Package):
+class PyPudb(PythonPackage):
     """Full-screen console debugger for Python"""
 
     homepage = "http://mathema.tician.de/software/pudb"
@@ -34,11 +34,6 @@ class PyPudb(Package):
 
     version('2016.2', '4573b70163329c1cb59836a357bfdf7c')
 
-    extends('python')
-
     depends_on('py-setuptools', type='build')
     depends_on('py-urwid@1.1.1:', type=('build', 'run'))
     depends_on('py-pygments@1.0:', type=('build', 'run'))
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))