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-py2cairo/package.py

@@ -26,19 +26,19 @@
 
 
 class PyPy2cairo(Package):
-    """bindings for the Cairo for Python 2,
-       to be used in Python."""
+    """Pycairo is a set of Python bindings for the cairo graphics library."""
 
-    homepage = "https://pypi.python.org/pypi/pycairo"
+    homepage = "https://www.cairographics.org/pycairo/"
     url      = "https://cairographics.org/releases/py2cairo-1.10.0.tar.bz2"
 
     version('1.10.0', '20337132c4ab06c1146ad384d55372c5')
 
     extends('python')
-    depends_on("cairo")
-    depends_on("pixman")
+
+    depends_on('cairo+X')
+    depends_on('pixman')
 
     def install(self, spec, prefix):
-        python('waf', 'configure', '--prefix=%s' % prefix)
+        python('waf', 'configure', '--prefix={0}'.format(prefix))
         python('waf', 'build')
         python('waf', 'install')