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-dbf/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyDbf(Package):
+class PyDbf(PythonPackage):
     """Pure python package for reading/writing dBase, FoxPro, and Visual FoxPro
     .dbf files (including memos)"""
 
@@ -33,8 +33,3 @@ class PyDbf(Package):
     url      = "https://pypi.python.org/packages/source/d/dbf/dbf-0.96.005.tar.gz"
 
     version('0.96.005', 'bce1a1ed8b454a30606e7e18dd2f8277')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)