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-mx/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyMx(Package):
+class PyMx(PythonPackage):
     """The eGenix.com mx Base Distribution for Python is a collection of
        professional quality software tools which enhance Python's
        usability in many important areas such as fast text searching,
@@ -36,8 +36,3 @@ class PyMx(Package):
     url      = "https://downloads.egenix.com/python/egenix-mx-base-3.2.8.tar.gz"
 
     version('3.2.8', '9d9d3a25f9dc051a15e97f452413423b')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)