Using hudston.util.Secret to encrypt the password instead of storing plain text

[pixman20]

Using hudston.util.Secret to encrypt the password so that it's not serialized in plain text to the config.xml

Note that this will break all existing jobs until the job is configured with the password.

[tomasbjerre]

Looks lite there are some failing tests. I can fix that after merge, or if you do it before.

Also I should mention that I will probably implement #5 for next release. And that will also break configuration. So no worries there.

[pixman20]

Looks like I overlooked the fact that hudson.util.Secret requires an active Jenkins instance to get the key for encrypting/decrypting.

I ended up noticing that the credentials binding plugin works with this plugin since it expands even the password environment variable. I'll end up using it that way, so the password will never be stored in plain text even as the plugin is now. It'll just show the environment variable I'm using in the config.xml, which is perfectly acceptable.

This PR can still be accepted, but it's not 100% necessary like I had thought so I'll leave it up to you to decide whether to bring it in or decline it. FWIW, if it's brought in it'll still work with the credentials plugin, but if it's not pulled in it's one less configuration change for active users.

Thanks for the awesome plugin!