Fixed issue #1478, #1599, #1709, motors being opposite, updated turning algorithm
Revert "Changed pins definition in variants from constants to #defines."

This reverts commit 7fcba37acfd11313640b3f5d5c813d63d2f59999.
Merge branch 'master' of github.com:X-Y/Arduino into X-Y-master
Added some friendly messages for Arduino Robot include errors on old sketches

After merging #1859, old sketches that uses Robot_Control library must include
Wire.h and SPI.h to work properly.