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-vcversioner/package.py

@@ -25,17 +25,12 @@
 from spack import *
 
 
-class PyVcversioner(Package):
+class PyVcversioner(PythonPackage):
     """Vcversioner: Take version numbers from version control."""
 
     homepage = "https://github.com/habnabit/vcversioner"
-    # base https://pypi.python.org/pypi/vcversioner/
     url      = "https://pypi.python.org/packages/source/v/vcversioner/vcversioner-2.16.0.0.tar.gz"
 
     version('2.16.0.0', 'aab6ef5e0cf8614a1b1140ed5b7f107d')
 
-    extends('python')
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))