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-simplegeneric/package.py

@@ -25,8 +25,8 @@
 from spack import *
 
 
-class PySimplegeneric(Package):
-    """Simple generic functions (similar to Python's own len(), 
+class PySimplegeneric(PythonPackage):
+    """Simple generic functions (similar to Python's own len(),
     pickle.dump(), etc.)"""
 
     homepage = "https://pypi.python.org/pypi/simplegeneric"
@@ -35,9 +35,4 @@ class PySimplegeneric(Package):
     version('0.8.1', 'f9c1fab00fd981be588fc32759f474e3')
     version('0.8', 'eaa358a5f9517a8b475d03fbee3ec90f')
 
-    extends('python')
-
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))