Added ssh-agent function to .bashrc_custom

Added an ssh-agent function provided publicly here http://www.cygwin.com/ml/cygwin/2001-06/msg00537.html

Step 2 of the github ssh key instructions recommend setting up a passphrase for your keys and using something like ssh-agent:
https://help.github.com/articles/generating-ssh-keys
This allows ubuntu to prompt you for a password for a session at login, but from there on will continue to allow password-less entry