added feature to store and retrieve password from keychain

rather than putting a password in a LaunchD or script this will allow using the System Keychain to store and retrieve so you would do something like this...  

to set the password
```shell
/usr/local/sbin/counterpart setpass
```
then on subsequent runs it could simply be this
```shell
/usr/local/sbin/counterpart -s "/" -d "/Volumes/Backup" -B
```