Add new methods for assigning and removing group to/from resources 

- hint: (one group, more resources)
- add method removeGroupFromResources
- add method assignGroupToResources
- add these method also to RPC
- methods are only cycles through existing methods removeGroupFromResource and assignGroupToResource in BlImpl
- fix rpc calling of method removeGroupsFromResource to use IDs instead of whole objects
- fix some javadoc
Merge pull request #155 from stavamichal/add-assign/remove-group-to/from-resources-methods

Add new methods for assigning and removing group to/from resources