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-pyelftools/package.py

@@ -25,15 +25,10 @@
 from spack import *
 
 
-class PyPyelftools(Package):
+class PyPyelftools(PythonPackage):
     """A pure-Python library for parsing and analyzing ELF files and DWARF
        debugging information"""
     homepage = "https://pypi.python.org/pypi/pyelftools"
     url      = "https://pypi.python.org/packages/source/p/pyelftools/pyelftools-0.23.tar.gz"
 
     version('0.23', 'aa7cefa8bd2f63d7b017440c9084f310')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)