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-prettytable/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyPrettytable(Package):
+class PyPrettytable(PythonPackage):
     """PrettyTable is a simple Python library designed to make
     it quick and easy to represent tabular data in visually
     appealing ASCII tables.
@@ -36,8 +36,4 @@ class PyPrettytable(Package):
 
     version('0.7.2', 'a6b80afeef286ce66733d54a0296b13b')
 
-    extends("python")
     depends_on("py-setuptools", type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)