cleanup holland BasePlugin interface

add class level attributes to cover:
    - author
    - summary
    - description
    - version
    - api_version
add plugin_namespace attribute to BasePlugin
set a default namespace for BackupPlugin
rename group -> namespace in plugin loaders
handle unset plugin attributes
implementing missing RegisteryPluginLoader.iterate method
add holland.core.exc module for standard holland errors
make holland.core.config:ConfigError inherit from HollandError
remove unused name attribute on Config class
report failing modules in test_runner
remove function = classmethod(function) workaround for py2.3

This was used for rhel4 support where decorator syntactic sugar
was not supported.  Currently there are no platforms with
python < 2.4 that holland 1.1 needs to support.