Added inverse writing mode "color"

Added an inverse "color" (XOR) to draw in reverse color over existing
pixels.
Macro for Wire.write to Wire.send in TWI

Current library does not compile in <1.0 because it references
Wire.write(), which was called Wire.send() in <1.0 -- added a macro to
wrap the change.
Delete Adafruit_SSD1306.h.bak
Merge pull request #16 from norpchen/master

Inverse XOR drawing mode and fix for pre 1.0 Wire.write()