refactor current_league helper method

to give priority to league slug in params over the one in session.
Fixes (https://github.com/stephanpavlovic/kickerapp/issues/80)
Merge branch 'master' into refactor_league_lookup

Conflicts:
	app/controllers/application_controller.rb
Merge pull request #83 from stephanpavlovic/refactor_league_lookup

refactor current_league helper method