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-pygobject/package.py

@@ -30,6 +30,8 @@ class PyPygobject(AutotoolsPackage):
        to be used in Python."""
 
     homepage = "https://pypi.python.org/pypi/pygobject"
+
+    # FIXME: This URL is no longer available for download from PyPi
     url      = "https://pypi.python.org/packages/6d/15/97c8b5ccca2be14cf59a2f79e15e3a82a1c3408a6b76b4107689a8b94846/pygobject-2.28.3.tar.bz2"
 
     version('2.28.3', 'aa64900b274c4661a5c32e52922977f9')