Add ability to define abstract options classes
Add ability to define abstract options classes

Only actual options should be added to the options dict of OptionMeta. This
patch adds a abstract kwarg to the OptionMeta class, that allows declare, if an
option is abstract and should therefore not be added.

agdsn/hades,agdsn/hades,agdsn/hades,agdsn/hades,agdsn/hades