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-jinja2/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyJinja2(Package):
+class PyJinja2(PythonPackage):
     """Jinja2 is a template engine written in pure Python. It provides
     a Django inspired non-XML syntax but supports inline expressions
     and an optional sandboxed environment."""
@@ -39,11 +39,6 @@ class PyJinja2(Package):
     version('2.7.1', '282aed153e69f970d6e76f78ed9d027a')
     version('2.7',   'c2fb12cbbb523c57d3d15bfe4dc0e8fe')
 
-    extends('python')
-
     depends_on('py-setuptools', type='build')
     depends_on('py-markupsafe', type=('build', 'run'))
     depends_on('py-babel@0.8:', type=('build', 'run'))  # optional, required for i18n
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))