Fix bugs when changing require setters of BITFeedbackManager after the setup

If the userName and userEmail delegates are implemented and the requireUserEmail or requireUserName are changed after that, then the user interface is shown even though the delegates define the values and the user shouldn't be able to change that.
Only push the user details view automatically onto the feedback list stack once at max

If the required manual user data is missing and the feedback list view appears the user detail view is automatically pushed. Now if the user cancels and goes back to the feedback list, it gets pushed again and so the user can never exit. Instead push this at maximum once. When providing feedback it will ask the missing user details again anyway.
Show proper missing name or email in the feedback list view button instead of "(null)" string
Merge branch 'develop' of github.com:bitstadium/HockeySDK-iOS into develop