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-coverage/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyCoverage(Package):
+class PyCoverage(PythonPackage):
     """ Testing coverage checker for python """
 
     homepage = "http://nedbatchelder.com/code/coverage/"
@@ -34,8 +34,3 @@ class PyCoverage(Package):
     version('4.0a6', '1bb4058062646148965bef0796b61efc')
 
     depends_on('py-setuptools', type='build')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)