:only and :except options can now take a single symbol as an argument

This is a really small patch so you can use the `:only` and `:except` options with a symbol like you can in the `routes.rb` file:

    resources :users, only: :show

I had some trouble running the tests because of the `001_create_activities.rb` file. I got this error:

    C:/Ruby193/lib/ruby/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/dependencies.rb:251:in `require': C:/.../public_activity/test/migrations/001_create_activities.rb:1: syntax error, unexpected tDOT2 (SyntaxError)
    ../../lib/generators/public_activity/migration/templates/migration.rb
      ^
    C:/.../public_activity/test/migrations/001_create_activities.rb:1: unknown regexp options - lb
      from C:/Ruby193/lib/ruby/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/dependencies.rb:251:in `block in require'
      from C:/Ruby193/lib/ruby/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/dependencies.rb:236:in `load_dependency'
      from C:/Ruby193/lib/ruby/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/dependencies.rb:251:in `require'
      from C:/Ruby193/lib/ruby/gems/1.9.1/gems/activerecord-3.2.9/lib/active_record/migration.rb:537:in `load_migration'
      from C:/Ruby193/lib/ruby/gems/1.9.1/gems/activerecord-3.2.9/lib/active_record/migration.rb:533:in `migration'
      from C:in `migrate'
      from C:/Ruby193/lib/ruby/gems/1.9.1/gems/activerecord-3.2.9/lib/active_record/migration.rb:720:in `block (2 levels) in migrate'

I tried to fix it but I just couldn't find a way to do it. So I simply solved the problem by copying the contents of the migration located in `/lib/generators/public_activity/migration/templates/migration.rb` into the `001_create_activities.rb`. The tests then passed so I also added 2 more tests to test this patch.

This is the first time I send a pull request so bear with me. I noticed you have several branches but I didn't know where to send the pull request so I just left it to the default master branch. If there's anything I can do to make this better, please say it.