Merge pull request #271 from jediunix/fix-8mhz-usb

Fix USB for 8mhz atmega32u4
Merge pull request #270 from jediunix/alarm-expires-in

Implement expires_in() for Alarm.
Add 8 MHz Pro Mini to boards.
Add .nfs.* to ignore
Merge branch 'master' of https://github.com/jediunix/Cosa into add-adafruit-sparkfun-boards
Last change to this branch.  Starting a new one...