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-emcee/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyEmcee(Package):
+class PyEmcee(PythonPackage):
     """emcee is an MIT licensed pure-Python implementation of Goodman & Weare's
     Affine Invariant Markov chain Monte Carlo (MCMC) Ensemble sampler."""
 
@@ -34,8 +34,4 @@ class PyEmcee(Package):
 
     version('2.1.0', 'c6b6fad05c824d40671d4a4fc58dfff7')
 
-    extends('python')
     depends_on('py-numpy', type=('build', 'run'))
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))