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-symengine/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PySymengine(Package):
+class PySymengine(PythonPackage):
     """Python wrappers for SymEngine, a symbolic manipulation library."""
 
     homepage = "https://github.com/symengine/symengine.py"
@@ -35,13 +35,11 @@ class PySymengine(Package):
     version('develop', git='https://github.com/symengine/symengine.py.git')
 
     # Build dependencies
-    extends('python')
     depends_on('python@2.7:2.8,3.3:')
     depends_on('py-setuptools',     type='build')
     depends_on('py-cython@0.19.1:')
     depends_on('cmake@2.8.7:',      type='build')
     depends_on('symengine@0.2.0:')
 
-    def install(self, spec, prefix):
-        python('setup.py', 'install', '--prefix=%s --symengine-dir=%s' %
-               (prefix, spec['symengine'].prefix))
+    def build_args(self, spec, prefix):
+        return ['--symengine-dir={0}'.format(spec['symengine'].prefix)]