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-netcdf/package.py

@@ -25,18 +25,14 @@
 from spack import *
 
 
-class PyNetcdf(Package):
+class PyNetcdf(PythonPackage):
     """Python interface to the netCDF Library."""
     homepage = "http://unidata.github.io/netcdf4-python"
     url      = "https://github.com/Unidata/netcdf4-python/tarball/v1.2.3.1rel"
 
     version('1.2.3.1', '4fc4320d4f2a77b894ebf8da1c9895af')
 
-    extends('python')
     depends_on('py-numpy', type=('build', 'run'))
     depends_on('py-cython', type=('build', 'run'))
     depends_on('py-setuptools', type=('build', 'run'))
     depends_on('netcdf')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)