Create time-limited / digits-count limited modes for memorization

[VivekThazhathattil]

As of now, there are no such options. It can be implemented as three different types of memorization - with a time limit, with number of digits limit or with no limits. Either one of the three modes should be activated before moving onto the memorization mode. The user should be free to choose a specific time limit / number of digits or not make a choice at all (in which case default value corresponding to the activated type is used).
In the 'ingame' fragment, check for the limiting conditions to decide whether to move to the recall mode or not. In the no limit mode, the user has to explicitly press 'recall' button to move to the recall mode.