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-autopep8/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyAutopep8(Package):
+class PyAutopep8(PythonPackage):
     """autopep8 automatically formats Python code to conform to the
     PEP 8 style guide."""
 
@@ -48,6 +48,3 @@ def url_for_version(self, version):
             return url.format('v', version)
         else:
             return url.format('ver', version)
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))