Add @ syntax to set command

Added the @ syntax to the set command for setting service options. It allows that the value for an option can be read out of a file:

juju set myservice myoption=@valuefile.txt

See also

https://bugs.launchpad.net/juju-core/+bug/1216967
Hehe, the Python version would have the same problem. Could be solved by "gmail" as file containing "@gmail.com" as content. ;)