mp3: added SPI routines plus simple testbed
mp3: added SPI routines plus simple testbed

vowstar/esp8266_mp3_decoder,vowstar/esp8266_mp3_decoder,HappyCodingRobot/esp8266_mp3_decoder,HappyCodingRobot/esp8266_mp3_decoder,vowstar/esp8266_mp3_decoder,chaosAD/esp8266_mp3_decoder,espressif/ESP8266_MP3_DECODER,espressif/ESP8266_MP3_DECODER,HappyCodingRobot/esp8266_mp3_decoder,chaosAD/esp8266_mp3_decoder,espressif/ESP8266_MP3_DECODER,chaosAD/esp8266_mp3_decoder