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-pexpect/package.py

@@ -25,16 +25,12 @@
 from spack import *
 
 
-class PyPexpect(Package):
+class PyPexpect(PythonPackage):
     """Pexpect allows easy control of interactive console applications."""
     homepage = "https://pypi.python.org/pypi/pexpect"
     url      = "https://pypi.io/packages/source/p/pexpect/pexpect-4.2.1.tar.gz"
 
     version('4.2.1', '3694410001a99dff83f0b500a1ca1c95')
+    version('3.3', '0de72541d3f1374b795472fed841dce8')
 
-    extends('python')
-    
-    depends_on('py-ptyprocess')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)
+    depends_on('py-ptyprocess', type=('build', 'run'))