Move creation of settings directory into helper function
Add new setter for global userID, userName, and userEmail properties

The values are used by the `BITCrashManager` to attach to a crash report and`BITFeedbackManager for assigning the user to a discussion thread.

The value can be set at any time and will be stored in the keychain on the current device only! To delete the value from the keychain set the value to `nil`.

These properties are optional and alternatives to the delegates. If you want to define specific values for each component, use the delegate instead which do overwrite the values set by these properties.

Also fixed a typo in the delegates documentation.