Allow session configuation

* add sessions DSL to configuration, used to enable sessions
* add Lotus::Config::Sessions responsible for sessions configuration
* move assets middleware configuration to Lotus::Config::Assets
* include Lotus::Action::Session to controllers if sessions are enabled
* defer loading default middleware stack (assets and sessions) to the
  middleware stack to the load time, and not middleware stack
  initialization.
* change Lotus::Config::Middleware tests to account for the
  configuration loading that happens at application initialization

Closes #2