Configure runtime Zope Component Architecture registry
        
        We need a 3 step process to make sure dependencies are met
        1. load static package-based ZCML files....standard stuff here.
        2. Register the config into the registry (i.e. make it available for
           lookup)
        3. Manually register zcml entries in the config (these entries 
           may be dependent on having the config available for lookup)
           
        Args:
            cache_config: application configuration file name or file object

def configure_zca(cls, cache_config):
        """Configure runtime Zope Component Architecture registry
        
        We need a 3 step process to make sure dependencies are met
        1. load static package-based ZCML files....standard stuff here.
        2. Register the config into the registry (i.e. make it available for
           lookup)
        3. Manually register zcml entries in the config (these entries 
           may be dependent on having the config available for lookup)
           
        Args:
            cache_config: application configuration file name or file object
        """
        # step 1
        packages = [sparc.apps.cache]
        Configure(packages)
        configure_vocabulary_registry()
        #step 2
        config = ElementTree.parse(cache_config).getroot()
        for zcml in config.findall('zcml'):
            zcml_file, package = 'configure.zcml' \
                        if 'file' not in zcml.attrib else zcml.attrib['file'],\
                            import_module(zcml.attrib['package'])
            zope.configuration.xmlconfig.XMLConfig(zcml_file, package)()
        #step3
        alsoProvides(config, IAppElementTreeConfig)
        sm = getSiteManager()
        sm.registerUtility(config, IAppElementTreeConfig)
        return config