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/python/package.py

@@ -99,6 +99,11 @@ def patch(self):
             r'\1setup.py\2 --no-user-cfg \3\6'
         )
 
+    @when('@:2.6,3.0:3.3')
+    def patch(self):
+        # See https://github.com/LLNL/spack/issues/1490
+        pass
+
     def install(self, spec, prefix):
         # TODO: The '--no-user-cfg' option for Python installation is only in
         # Python v2.7 and v3.4+ (see https://bugs.python.org/issue1180) and