Added tests for models

 - Refactored models a little
 - Added tests for Plan(uncompleted) and User
 - Added in-memory-db settings for testing
 - Removed IntegrationSpec (doesn't work with flyway plugin though I've not investigated in detail)
 - Added scalikejdbc-mapper-generator (can get test template by executing `sbt scalikejdbc-gen-echo user`)