Add hardware SPI

This PR is a rewrite of the library based on the recent ST7735 codebase as of commit adafruit/Adafruit-ST7735-Library@98c5d9d1a1899ba6a6cd3d8f5745ae87a1bca29f.

Features:
  - hardware SPI 
  - add support for Cortex-M3 Arduinos (similar to the ST7735 library)
  - PCD8544 displays can be used as drop-in replacements for ST7735-based displays

Included sample code has been tested on Arduino Nano.

Not yet implemented is the ST7735's command-list architecture which could also be ported.

If there is general interest in this patch I propose to push another one that extracts all SPI-specific code into an Adafruit_GFX_SPI class that can be the common basis for all/most Adafruit TFT libraries to simplify maintenance.