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-biopython/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyBiopython(Package):
+class PyBiopython(PythonPackage):
     """A distributed collaborative effort to develop Python libraries and
        applications which address the needs of current and future work in
        bioinformatics.
@@ -36,9 +36,5 @@ class PyBiopython(Package):
 
     version('1.65', '143e7861ade85c0a8b5e2bbdd1da1f67')
 
-    extends('python')
     depends_on('py-mx', type=('build', 'run'))
     depends_on('py-numpy', type=('build', 'run'))
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)