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/udunits2/package.py

@@ -29,9 +29,9 @@ class Udunits2(AutotoolsPackage):
     """Automated units conversion"""
 
     homepage = "http://www.unidata.ucar.edu/software/udunits"
-    url      = "ftp://ftp.unidata.ucar.edu/pub/udunits/udunits-2.2.20.tar.gz"
+    url      = "ftp://ftp.unidata.ucar.edu/pub/udunits/udunits-2.2.21.tar.gz"
 
-    version('2.2.20', '1586b70a49dfe05da5fcc29ef239dce0')
+    version('2.2.21', '1f6d3375efc1f124790a4efb7102cdb7')
 
     depends_on('expat')