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-cffi/package.py

@@ -22,12 +22,10 @@
 # License along with this program; if not, write to the Free Software
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import os
-
 from spack import *
 
 
-class PyCffi(Package):
+class PyCffi(PythonPackage):
     """Foreign Function Interface for Python calling C code"""
     homepage = "http://cffi.readthedocs.org/en/latest/"
     # base https://pypi.python.org/pypi/cffi
@@ -35,18 +33,15 @@ class PyCffi(Package):
 
     version('1.1.2', 'ca6e6c45b45caa87aee9adc7c796eaea')
 
-    extends('python')
     depends_on('py-setuptools', type='build')
     depends_on('py-pycparser', type=('build', 'run'))
     depends_on('libffi')
 
-    def install(self, spec, prefix):
+    def setup_environment(self, spack_env, run_env):
         # This sets the compiler (and flags) that distutils will use
         # to create the final shared library.  It will use the
         # compiler specified by the environment variable 'CC' for all
         # other compilation.  We are setting the 'LDSHARED" to the
         # spack compiler wrapper plus a few extra flags necessary for
         # building the shared library.
-        os.environ['LDSHARED'] = "{0} -shared -pthread".format(spack_cc)
-
-        setup_py('install', '--prefix=%s' % prefix)
+        spack_env.set('LDSHARED', "{0} -shared -pthread".format(spack_cc))