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-xlrd/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyXlrd(Package):
+class PyXlrd(PythonPackage):
     """Library for developers to extract data from Microsoft Excel (tm)
     spreadsheet files"""
 
@@ -33,8 +33,3 @@ class PyXlrd(Package):
     url      = "https://pypi.python.org/packages/source/x/xlrd/xlrd-0.9.4.tar.gz"
 
     version('0.9.4', '911839f534d29fe04525ef8cd88fe865')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)