Fix for Arduino.h #include statements

The change we talked about on Twitter:

Comment:
Changing all #include statements of "arduino.h" to "Arduino.h" so the file can be found on case sensitive file systems.

I'm curious, if you go into Disk Utility on your Mac, does it show your disk format as "Mac OS Extended (Journaled)" or "Mac OS Extended (Case-sensitive, Journaled)"?