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-pycparser/package.py

@@ -25,15 +25,11 @@
 from spack import *
 
 
-class PyPycparser(Package):
+class PyPycparser(PythonPackage):
     """A complete parser of the C language, written in pure python."""
     homepage = "https://github.com/eliben/pycparser"
     url      = "https://pypi.python.org/packages/source/p/pycparser/pycparser-2.13.tar.gz"
 
     version('2.13', 'e4fe1a2d341b22e25da0d22f034ef32f')
 
-    extends('python')
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)