Support boards with only SPI or Wire (I2C)

[caternuson]

This library currently requires both SPI and Wire to be present for any target board. This can cause build failures for targets that only have one or the other. Ex:

adafruit/Adafruit_LiquidCrystal#15
adafruit/Adafruit_Learning_System_Guides#2340

Need to add some preproc or other logic to protect against this and allow these type of targets to be used.