Loader: only create namespaces if they don't already exist

[GavinJoyce]

This will avoid these warnings:

/Users/gavin/.rvm/gems/ruby-2.0.0-p247/gems/restpack_service-0.0.40/lib/restpack_service/loader.rb:31: warning: already initialized constant Models
/Users/gavin/.rvm/gems/ruby-2.0.0-p247/gems/restpack_service-0.0.40/lib/restpack_service/loader.rb:31: warning: previous definition of Models was here
/Users/gavin/.rvm/gems/ruby-2.0.0-p247/gems/restpack_service-0.0.40/lib/restpack_service/loader.rb:31: warning: already initialized constant Serializers
/Users/gavin/.rvm/gems/ruby-2.0.0-p247/gems/restpack_service-0.0.40/lib/restpack_service/loader.rb:31: warning: previous definition of Serializers was here
/Users/gavin/.rvm/gems/ruby-2.0.0-p247/gems/restpack_service-0.0.40/lib/restpack_service/loader.rb:31: warning: already initialized constant Commands
/Users/gavin/.rvm/gems/ruby-2.0.0-p247/gems/restpack_service-0.0.40/lib/restpack_service/loader.rb:31: warning: previous definition of Commands was here