Add random generator and key repeat

CHANGELOG:

• Now left and right can be keep pressed to repeat actions
• Random generator makes sure that next games will not be so
  deterministic. Random seed is provided by the proper user moves.

v0.2.0: Bug fixes and general improvement

We have a new contributor, my Microprocessors professor Marcelo
M. S. Souza.

This release fix properly problems with re-entrance of interruptions.
Use timers instead pooling for fall_one_row part and another improvements.

The changelog provided by Marcelo was:

• Implemented round robin schedule.
• Implemented a general check_collission() to eliminate
  check_fall_collision(), check_right_collision() and
  check_left_collission().
• Pieces matrices resized to 3x3.
• Improved piece rotation algorithm.
• Added Timer2 int for display refresh.
• Added key_manager() to deal with key reactions outside external interrupts.
• Added game_manager() to implement game as a state machine.
• Implemented Gameover animation inline in game_manager().
• Added timer variable for timming management.
• Instead of disable all interrupts when updating display buffer,
  disable Timer2 int only.
• Implemented init() routine and moved all initializations into it.
• Changed either hw and sw strategy to handle display colummns.

Authors & Contributors:

• Manoel Vilela
• Marcelo M. S. Souza

v0.1.0: First version release!

The firmware currently contains all the basic systems for simulating
tetris. However there is some bugs in interruptions and fall_one_row
procedure.

This version contains as well a hardware schematics, the input control
for left/right and a naive version trying to rotate a tetris block.
(using transposition instead rotating properly)

The full row detection is working fine and cleaning as well.
The RNG method for next block selection is totally lame and
this should be change in future releases.

I hope you enjoy it.

Authors & Contributors:

• Manoel Vilela