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-mock/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyMock(Package):
+class PyMock(PythonPackage):
     """mock is a library for testing in Python. It allows you to replace parts
     of your system under test with mock objects and make assertions about how
     they have been used."""
@@ -35,9 +35,5 @@ class PyMock(Package):
 
     version('1.3.0', '73ee8a4afb3ff4da1b4afa287f39fdeb')
 
-    extends('python')
     depends_on('py-pbr', type=('build', 'run'))
     depends_on('py-setuptools@17.1:', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)