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-nose/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyNose(Package):
+class PyNose(PythonPackage):
     """nose extends the test loading and running features of unittest,
     making it easier to write, find and run tests."""
 
@@ -36,8 +36,4 @@ class PyNose(Package):
     version('1.3.6', '0ca546d81ca8309080fc80cb389e7a16')
     version('1.3.7', '4d3ad0ff07b61373d2cefc89c5d0b20b')
 
-    extends('python')
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)