Added Some Sequential Circuits

1. Rewrote `SR` and `D`
2. Created `JK` and `T`
3. Created a new file [ `counters.py` ]
4. Added `Counters` Class as base for all counters.
5. Using `Counters` Class, created two counters `BinaryCounter` and `NBitRippleCounter` [can have N bits - N is the variable that will be passed as a parameter]
6. Created tests for all the FlipFlops and 2 Counters.