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-sphinx/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PySphinx(Package):
+class PySphinx(PythonPackage):
     """Sphinx Documentation Generator."""
     homepage = "http://sphinx-doc.org"
     url      = "https://pypi.python.org/packages/source/S/Sphinx/Sphinx-1.3.1.tar.gz"
@@ -49,6 +49,3 @@ class PySphinx(Package):
     depends_on('py-alabaster@0.7:',          type=('build', 'run'))
     depends_on('py-imagesize', when='@1.4:', type=('build', 'run'))
     depends_on('py-sphinx-rtd-theme@0.1:',   type=('build', 'run'))  # optional as of 1.4
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))