Documentation improvements
Add support for background fetch in BITCrashManager

Automatic sending crash reports or alert asking the user for permission to send crash reports will only appear if the app is running in the foreground
Add support for background fetch in BITFeedbackManager

Fetching of new messages and showing an alert will only be done if the app is in the foreground
Add support for background fetch in BITStoreUpdateManager

Checking for new app store updates and showing an alert will only be done if the app is in the foreground
Add support for background fetch in BITUpdateManager

Checks for updates will only be done if the app is in the foreground. This is just for double safety, since the BITAuthenticator already reports that the app is not validated in this case