New method deleteGroups

- delete list of groups sorted by name (reverse) 
- use existing deleteGroup methods with force parameter
- add tests for this new method
- add method to RPC
Use native reverseComparation instead of multiplication by -1.
Merge pull request #288 from stavamichal/deleteGroups

New method deleteGroups

 - delete list of groups sorted by name (reverse)
 - use existing deleteGroup methods with force parameter
 - add tests for this new method
 - add method to RPC