Feature/add 7 segment display

[mateusz-sz]

This pull requests introduces new element to the digitaljs package. A classical 7 segment display, that takes 8 bit input.

The most significant bit controls the decimal point dot. The other bits control segments of the display as follows: 1 -> a, 2 -> b, 3 -> c, 4 -> d, 5 -> e, 6 -> f, 7 -> g

Bit set to 1 fires up the "LED".

[tilk]

Didn't test it yet, but the code looks fine.