Byron/security key

Add Alert Dialog prompting user for a security key to access the rest of the app (as an added security feature).
- Alert Dialog appears after salesforce login and before the main activity view is launched. 
- Alert Dialog only appears when the security key that is stored in SharedPreferences is either null or no longer correct.
- If the user enters the incorrect key, the dialog will trigger a cycle of "error" and "retry" dialogs until the right key is entered.
- When the correct security key is entered, it is written to SharedPreferences so that the user will not have to reenter the key again (unless the key is changed.)