Adding support to authentication with Putty Agent

Hi,

At beginning I want to say: thank you for this plugin. This tool is very useful for me.
I added support to authentication with Putty Agent. If you are interested to use these changes, you can do that (check firstly of course this solution). I will be honored. :-)

And here is a simple example of usage (I assume that the private key is added to Putty Agent):

```groovy
apply plugin: 'ssh'

remotes {
	example {
		host = 'example.com'
		port = 22
		user = 'username'
		usePuttyAgent = true
	}
}

ssh {
	knownHosts = allowAnyHosts
}

	
buildscript {
	repositories {
		mavenCentral()
		dependencies {
			classpath 'org.hidetake:gradle-ssh-plugin:0.2.6'
		}
	}
}

task sshtest() << {
	sshexec {				
		session(remotes.example) {
			execute('ls -la')
		}
	}
}
```

Greetings,
Jarek