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-protobuf/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyProtobuf(Package):
+class PyProtobuf(PythonPackage):
     """Protocol buffers are Google's language-neutral, platform-neutral,
     extensible mechanism for serializing structured data - think XML, but
     smaller, faster, and simpler. You define how you want your data to be
@@ -42,9 +42,4 @@ class PyProtobuf(Package):
     version('2.4.1', '72f5141d20ab1bcae6b1e00acfb1068a')
     version('2.3.0', 'bb020c962f252fe81bfda8fb433bafdd')
 
-    extends('python')
-
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))