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 input key is incorrect into the alert dialog, the dialog will trigger a cycle of "error" and "retry" dialogs until the right key is entered.
- When correct security key is entered, writes security key to SharedPreferences so that the user will not have to reenter the key as long as the key is not changed. 

Best class description NA