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-pathspec/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyPathspec(Package):
+class PyPathspec(PythonPackage):
     """pathspec extends the test loading and running features of unittest,
     making it easier to write, find and run tests."""
 
@@ -34,8 +34,4 @@ class PyPathspec(Package):
     version('0.3.4', '2a4af9bf2dee98845d583ec61a00d05d',
         url='https://pypi.python.org/packages/14/9d/c9d790d373d6f6938d793e9c549b87ad8670b6fa7fc6176485e6ef11c1a4/pathspec-0.3.4.tar.gz')
 
-    extends('python')
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)