feature/login: move log in to the time right after user launches  our app.
feature/login: log in screen UI change 1. remove logout button 2. add a friendly label  3. other UI adjustment
Merge pull request #16 from google/feature/login

Feature/login
issue17: disallow user to join an experiment multiple times
Merge pull request #18 from google/issue17

issue17: disallow user to join an experiment multiple times
Merge branch 'ios-init' of github.com:google/paco into issue313

Conflicts:
	Paco-iOS/Paco/source/core/client/PacoClient.h
Merge branch 'issue313' of github.com:google/paco into issue313