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-proj/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyProj(Package):
+class PyProj(PythonPackage):
     """Python interface to the PROJ.4 Library."""
     homepage = "http://jswhit.github.io/pyproj/"
     url      = "https://github.com/jswhit/pyproj/tarball/v1.9.5.1rel"
@@ -37,8 +37,6 @@ class PyProj(Package):
 
     version('1.9.5.1', 'a4b80d7170fc82aee363d7f980279835')
 
-    extends('python')
-
     depends_on('py-cython', type='build')
     depends_on('py-setuptools', type='build')
 
@@ -46,6 +44,3 @@ class PyProj(Package):
     # The py-proj git repo actually includes the correct version of PROJ.4,
     # which is built internally as part of the py-proj build.
     # Adding depends_on('proj') will cause mysterious build errors.
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)