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-numexpr/package.py

@@ -25,16 +25,16 @@
 from spack import *
 
 
-class PyNumexpr(Package):
+class PyNumexpr(PythonPackage):
     """Fast numerical expression evaluator for NumPy"""
     homepage = "https://pypi.python.org/pypi/numexpr"
     url      = "https://pypi.python.org/packages/source/n/numexpr/numexpr-2.4.6.tar.gz"
 
+    version('2.6.1', '6365245705b446426df9543ad218dd8e',
+            url="https://pypi.python.org/packages/c6/f0/11628fa4d332d8fe9ab0ba8e9bfe0e065fb6b5324859171ee72d84e079c0/numexpr-2.6.1.tar.gz")
+    version('2.5',   '84f66cced45ba3e30dcf77a937763aaa')
     version('2.4.6', '17ac6fafc9ea1ce3eb970b9abccb4fbd')
-    version('2.5', '84f66cced45ba3e30dcf77a937763aaa')
 
-    extends('python')
-    depends_on('py-numpy', type=('build', 'run'))
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)
+    depends_on('python@2.6:')
+    depends_on('py-numpy@1.6:', type=('build', 'run'))
+    depends_on('py-setuptools', type='build')