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-twisted/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyTwisted(Package):
+class PyTwisted(PythonPackage):
     """An asynchronous networking framework written in Python"""
     homepage = "https://twistedmatrix.com/"
     url      = "https://pypi.python.org/packages/source/T/Twisted/Twisted-15.3.0.tar.bz2"
@@ -34,8 +34,3 @@ class PyTwisted(Package):
     version('15.3.0', 'b58e83da2f00b3352afad74d0c5c4599')
 
     depends_on('py-setuptools', type='build')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)