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-csvkit/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyCsvkit(Package):
+class PyCsvkit(PythonPackage):
     """A library of utilities for working with CSV, the king of tabular file
     formats"""
 
@@ -34,14 +34,9 @@ class PyCsvkit(Package):
 
     version('0.9.1', '48d78920019d18846933ee969502fff6')
 
-    extends('python')
-
     depends_on('py-dateutil', type=('build', 'run'))
     depends_on('py-dbf', type=('build', 'run'))
     depends_on('py-xlrd', type=('build', 'run'))
     depends_on('py-sqlalchemy', type=('build', 'run'))
     depends_on('py-six', type=('build', 'run'))
     depends_on('py-openpyxl', type=('build', 'run'))
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)