added new encoder library under arduino_src/code/Nano_encoder
added definitions file and added to other lib_encoder files
added definitions file
enoder library reads using interrupts. Tested with pushbutton. Closes #35
Merge pull request #48 from CMU-Robotics-Club/arathorn593_nano_encoder

Arathorn593 nano encoder. Created the Encoder Library. Closes #35. Tested in 'unit test' hardware rig with push button. To do: Implement with main Nano Code