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-six/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PySix(Package):
+class PySix(PythonPackage):
     """Python 2 and 3 compatibility utilities."""
 
     homepage = "https://pypi.python.org/pypi/six"
@@ -37,6 +37,3 @@ class PySix(Package):
     extends('python', ignore=r'bin/pytest')
 
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))