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-virtualenv/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyVirtualenv(Package):
+class PyVirtualenv(PythonPackage):
     """virtualenv is a tool to create isolated Python environments."""
     homepage = "http://virtualenv.readthedocs.org/projects/virtualenv/"
     url      = "https://pypi.python.org/packages/source/v/virtualenv/virtualenv-1.11.6.tar.gz"
@@ -34,8 +34,4 @@ class PyVirtualenv(Package):
     version('13.0.1', '1ffc011bde6667f0e37ecd976f4934db')
     version('15.0.1', '28d76a0d9cbd5dc42046dd14e76a6ecc')
 
-    extends('python')
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)