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-enum34/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyEnum34(Package):
+class PyEnum34(PythonPackage):
     """Python 3.4 Enum backported to 3.3, 3.2, 3.1, 2.7, 2.6, 2.5, and 2.4."""
 
     homepage = "https://pypi.python.org/pypi/enum34"
@@ -34,12 +34,10 @@ class PyEnum34(Package):
     version('1.1.6', '5f13a0841a61f7fc295c514490d120d0',
             url="https://pypi.python.org/packages/bf/3e/31d502c25302814a7c2f1d3959d2a3b3f78e509002ba91aea64993936876/enum34-1.1.6.tar.gz")
 
-    extends('python')
     depends_on('python@2.4:2.8,3.3:')
 
-    depends_on('py-ordereddict', when='^python@:2.6.999', type=('build', 'run'))
+    # This dependency breaks concretization
+    # See https://github.com/LLNL/spack/issues/2793
+    # depends_on('py-ordereddict', when='^python@:2.6.999', type=('build', 'run'))  # noqa
 
     depends_on('py-setuptools', type='build')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))