From LedHed's Wiki
Hard Reset Switch
Raspberry Pi B, B+ and 2 have P6/Run headers that can be used to reset the Pi or power it on from halt. Note: The location of this pin header varies by hardware revision.
Soft Reset/Halt
You can also use GPIO Pin 5 & 6 along with some software that will monitor for a low state and then run a command. You can get the script here Set the script to run at start by adding it to /etc/crontab
@reboot root /home/user/scripts/raspi_gpio_actions.sh
Disclaimer: I did not write this script, I'm regurgitating her for my own reference. For full details see the reference section.