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-pytz/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyPytz(Package):
+class PyPytz(PythonPackage):
     """World timezone definitions, modern and historical."""
 
     homepage = "https://pypi.python.org/pypi/pytz"
@@ -37,9 +37,4 @@ class PyPytz(Package):
     version('2015.4', '417a47b1c432d90333e42084a605d3d8')
     version('2016.3', 'abae92c3301b27bd8a9f56b14f52cb29')
 
-    extends('python')
-
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))