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-openpyxl/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyOpenpyxl(Package):
+class PyOpenpyxl(PythonPackage):
     """A Python library to read/write Excel 2007 xlsx/xlsm files"""
 
     homepage = 'http://openpyxl.readthedocs.org/'
@@ -33,10 +33,5 @@ class PyOpenpyxl(Package):
 
     version('2.4.0-a1', 'e5ca6d23ceccb15115d45cdf26e736fc')
 
-    extends('python')
-
     depends_on('py-jdcal', type=('build', 'run'))
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)