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-matplotlib/package.py

@@ -26,7 +26,7 @@
 import os
 
 
-class PyMatplotlib(Package):
+class PyMatplotlib(PythonPackage):
     """matplotlib is a python 2D plotting library which produces publication
     quality figures in a variety of hardcopy formats and interactive
     environments across platforms."""
@@ -80,7 +80,7 @@ class PyMatplotlib(Package):
 
     # --------- Optional dependencies
     depends_on('pkg-config', type='build')    # why not...
-    depends_on('py-pillow', when='+image', type=('build', 'run'))
+    depends_on('pil', when='+image', type=('build', 'run'))
     depends_on('py-ipython', when='+ipython')
     depends_on('ghostscript', when='+latex', type='run')
     depends_on('texlive', when='+latex', type='run')
@@ -95,9 +95,10 @@ class PyMatplotlib(Package):
     # depends_on('ttconv')
     depends_on('py-six@1.9.0:', type=('build', 'run'))
 
-    def install(self, spec, prefix):
-        setup_py('build')
-        setup_py('install', '--prefix={0}'.format(prefix))
+    @PythonPackage.sanity_check('install')
+    def set_backend(self):
+        spec = self.spec
+        prefix = self.prefix
 
         if '+qt' in spec or '+tk' in spec:
             # Set backend in matplotlib configuration file