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-sphinx-rtd-theme/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PySphinxRtdTheme(Package):
+class PySphinxRtdTheme(PythonPackage):
     """ReadTheDocs.org theme for Sphinx."""
 
     homepage = "https://pypi.python.org/pypi/sphinx_rtd_theme"
@@ -34,9 +34,4 @@ class PySphinxRtdTheme(Package):
     version('0.1.10a0', '83bd95cae55aa8b773a8cc3a41094282',
             url="https://pypi.python.org/packages/da/6b/1b75f13d8aa3333f19c6cdf1f0bc9f52ea739cae464fbee050307c121857/sphinx_rtd_theme-0.1.10a0.tar.gz")
 
-    extends('python')
-
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))