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-backports-shutil-get-terminal-size/package.py

@@ -25,8 +25,8 @@
 from spack import *
 
 
-class PyBackportsShutilGetTerminalSize(Package):
-    """A backport of the get_terminal_size function 
+class PyBackportsShutilGetTerminalSize(PythonPackage):
+    """A backport of the get_terminal_size function
     from Python 3.3's shutil."""
 
     homepage = "https://pypi.python.org/pypi/backports.shutil_get_terminal_size"
@@ -34,10 +34,5 @@ class PyBackportsShutilGetTerminalSize(Package):
 
     version('1.0.0', '03267762480bd86b50580dc19dff3c66')
 
-    extends('python')
-
     depends_on('py-setuptools', type='build')
     depends_on('python@:3.2.999')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))