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-gnuplot/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyGnuplot(Package):
+class PyGnuplot(PythonPackage):
     """Gnuplot.py is a Python package that allows you to create graphs from
        within Python using the gnuplot plotting program."""
     homepage = "http://gnuplot-py.sourceforge.net/"
@@ -33,8 +33,4 @@ class PyGnuplot(Package):
 
     version('1.8', 'abd6f571e7aec68ae7db90a5217cd5b1')
 
-    extends('python')
     depends_on('py-numpy', type=('build', 'run'))
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)