Rename withDefault: getter methods to fallback:

For example:

    -(NSDate *)dateForKey:(NSString *)variableName withDefault:(NSDate *)defaultValue;

changed to:

    -(NSDate *)dateForKey:(NSString *)variableName fallback:(NSDate *)fallbackValue;

Signed-off-by: Miguel M. Almeida <miguelcma@CMA-Retina.local>
Update Readme with fallback values

Signed-off-by: Miguel M. Almeida <miguelcma@CMA-Retina.local>