Add Style/SelfAssignment Rubocop cop

The cop enforces use of self assignment operator E.g. a=a+2 gets
written as a+=2. Also the offenses listed in rubocop.todo.yml
have been corrected automatically with the --auto-correct option.
Fixes issue #1531

.rubocop_todo.yml

@@ -721,14 +721,6 @@ Style/RegexpLiteral:
   Exclude:
     - 'Guardfile'
 
-# Offense count: 3
-# Cop supports --auto-correct.
-Style/SelfAssignment:
-  Exclude:
-    - 'app/models/event.rb'
-    - 'db/migrate/20141104131625_generate_username.rb'
-    - 'spec/support/save_feature_failures.rb'
-
 # Offense count: 3
 # Cop supports --auto-correct.
 # Configuration parameters: EnforcedStyle, SupportedStyles.