add focus group to isolate examples and ensure default group is run if no command line overrides

https://www.relishapp.com/rspec/rspec-core/v/2-12/docs/command-line/tag-option 
to run the focus specs use: 
bundle exec guard -g focus --no-interactions
Merge pull request #57 from camallen/add_focus_group_guard_rspec

update guardfile to provide rspec focus