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-ply/package.py

@@ -25,14 +25,9 @@
 from spack import *
 
 
-class PyPly(Package):
+class PyPly(PythonPackage):
     """PLY is nothing more than a straightforward lex/yacc implementation."""
     homepage = "http://www.dabeaz.com/ply"
     url      = "http://www.dabeaz.com/ply/ply-3.8.tar.gz"
 
     version('3.8', '94726411496c52c87c2b9429b12d5c50')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)