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-scientificpython/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyScientificpython(Package):
+class PyScientificpython(PythonPackage):
     """ScientificPython is a collection of Python modules for
        scientific computing. It contains support for geometry,
        mathematical functions, statistics, physical units, IO,
@@ -36,8 +36,3 @@ class PyScientificpython(Package):
     version('2.8.1', '73ee0df19c7b58cdf2954261f0763c77')
 
     depends_on('py-numpy')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)