newly implemented modules:
(1) free_buffer_tracker.v
(2) memory_bank.v
(3) testbench.v (for free_buffer_tracker module)

We also modified the Makefile to support the automatically simulation and results nevagation.
the 'make run' command will first compile the source code with vcs, then run the simulation. During the simulation run, 'debug.vcd' file was generated. Finally, gtkwave is invoked to browse the debug.vcd file.