Add 4- & 8-bit parallel interfaces

[rm-hull]

Somewhat related to rm-hull/luma.lcd#19

Use RPi.GPIO to bit-bang out 4 or 8 bus lines according to the HD44780 datasheet spec. The interface should have the same "shape" as the existing luma.core.interface.serial interfaces - i.e. implement data() and command() methods.

Also: Move luma.core.serial to luma.core.interface.serial and add this new functionality to luma.core.interface.parallel. Various breaking changes to device implementations to rename serial_interface= constructor arg to interface=. (done with #78)

(screen grab from https://cdn-shop.adafruit.com/datasheets/HD44780.pdf)