Issue #57 : add placeholder files
Issue #57 : implement PacoExpressionExecutor
Issue #57 : code refactoring
Issue #57 : remove legacy comment, code reorganizing
Issue #57 : code refactoring by defining enums for reponseType
Issue #57 : extract condition evaluation logic to PacoInputEvaluator
Issue #57 : extract condition evaluation logic to PacoInputEvaluator and PaocExpressionExecutor
Issue #57 : replace the old evaluation function
Issue #57 : fix tagging inputs' dependency
Issue #57 : fix the conditional logic for list
Issue #57 : fix the validation for non-multi-selectable list
Issue #57 : remove legacy code, comment out log for debugging
Issue #57: fix a type
Merge pull request #140 from google/issue57

Issue57
iOS: send UI update to main thread queue to avoid potential crash
Issue #91 : move user account storage from PacoClient to PacoAuthenticator
Issue #91 : move detailed log in logic from PacoMainViewController to PacoClient
Issue #91 : if a valid cookie exists, no need to log in
Issue #91 : hide loginWithClientLogin as an internal method
Issue #91 : code refactor to improve