Add remove methods for group_resource and group attributes

- add method removeAttributes for group and resource able to remove also group attributes
- add method removeAllAttributes for group and resource able to remove also all group attributes
- add these methods also to RPC
- add simple tests for those two methods
Merge pull request #212 from stavamichal/removeAttributes_for_group_and_group_resource_attributes

- Add method removeAttributes for group and resource able to remove also group attributes
- Add method removeAllAttributes for group and resource able to remove also all group attributes
- Add these methods also to RPC
- Add simple tests for those two methods