Requires iOS 8
Also available, SafariAlwaysPrivate (iOS 7)
Very simple tweak that when enabled, forces Safari to always be in "Private Browsing Mode". You will also be unable to leave Private Browsing mode until the tweak is disabled & Safari relaunched
No respring required, close Safari (if open) from the switcher view , then relaunch!
If you require this for iOS7 please look for SafariAlwaysPrivate.
Enable from Settings.