Support .pem files

[arnuschky]

Currently, to use the keyfiles provided from EC2, one needs to do the following step:

ssh-keygen -f ~/.ssh/mykey.pem -y > ~/.ssh/mykey.pem.pub
credulous save -k ~/.ssh/mykey.pem.pub

Would be great if this could be automated. Minor feature though, as above command is a usable workaround (partly putting it here for documentation purposes).