feat(osx): improve support for OSX

[mgcrea]

Better to use the up-to-date teensy3 files directly on OSX.

[apmorton]

I feel like this kinda defeats the purpose of using this project template.

What happens when a few months down the line you go to rebuild your project, but the teensy core has changed in the arduino app? Your code no longer compiles, even though you are carrying the entire teensy core around with you in the git repo.

It might be better to make this optional, or simply include it as a comment

# path location for Teensy 3 core
COREPATH = teensy3

# uncomment the below line on mac to use the shared teensy core in Arduino.app
#COREPATH = /Applications/Arduino.app/Contents/Resources/Java/hardware/teensy/cores/teensy3

[apmorton]

another option I just saw in the network graph, pretty neat. https://github.com/moonfall/teensy-template/commit/738e56f029a08ed3f5fb80c44ba38d1504df6f40

Instead of modifying the Makefile at all, just symlink the core/tools directory.