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-scikit-learn/package.py

@@ -25,8 +25,8 @@
 from spack import *
 
 
-class PyScikitLearn(Package):
-    """"""
+class PyScikitLearn(PythonPackage):
+    """A set of python modules for machine learning and data mining."""
     homepage = "https://pypi.python.org/pypi/scikit-learn"
     url      = "https://pypi.python.org/packages/source/s/scikit-learn/scikit-learn-0.15.2.tar.gz"
 
@@ -34,11 +34,6 @@ class PyScikitLearn(Package):
     version('0.16.1', '363ddda501e3b6b61726aa40b8dbdb7e')
     version('0.17.1', 'a2f8b877e6d99b1ed737144f5a478dfc')
 
-    extends('python')
-
     depends_on('py-setuptools', type='build')
     depends_on('py-numpy', type=('build', 'run'))
     depends_on('py-scipy', type=('build', 'run'))
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)