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-sympy/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PySympy(Package):
+class PySympy(PythonPackage):
     """SymPy is a Python library for symbolic mathematics."""
     homepage = "https://pypi.python.org/pypi/sympy"
     url      = "https://pypi.python.org/packages/source/s/sympy/sympy-0.7.6.tar.gz"
@@ -33,8 +33,4 @@ class PySympy(Package):
     version('0.7.6', '3d04753974306d8a13830008e17babca')
     version('1.0', '43e797de799f00f9e8fd2307dba9fab1')
 
-    extends('python')
     depends_on('py-mpmath', when='@1.0:')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)