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-ordereddict/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyOrdereddict(Package):
+class PyOrdereddict(PythonPackage):
     """A drop-in substitute for Py2.7's new collections.
     OrderedDict that works in Python 2.4-2.6."""
 
@@ -33,9 +33,3 @@ class PyOrdereddict(Package):
     url      = "https://pypi.python.org/packages/source/o/ordereddict/ordereddict-1.1.tar.gz"
 
     version('1.1', 'a0ed854ee442051b249bfad0f638bbec')
-
-    extends('python')
-    depends_on('python@2.4:2.6.999')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))