don't start the RESTGateway if given disable_http: true
refactor actor retrieval logic in Mixin::Services

now has no knowledge about the classes defining the actors so we
don't need to load code for actors we don't want to run
add Mixin::Services#cache_builder
abstract generic actor/server behavior into GenericServer module
Merge pull request #31 from RiotGames/load-when-necessary

Only load RESTGateway when required