Windows slave support

This changeset introduces support for windows AMI to the ec2-plugin.
The windows slaves are connected to with SMB over TCP to send the initial
slave.jar and connected to from jenkins using WinRM/WinRS.

Unlile linux based AMI, the windows AMI requires

* a specific security group allowing SMB over TCP (incoming TCP port 445)
* a specific securify group alloaing WinRM (incoming TCP port 5985)
* a java virtual machine installed in the Windows %PATH%
* SMB over TCP should be enabled in the windows firewall (this can be
done with: netsh firewall set portopening tcp 445 smb enable)
* WinRM must be configured and enabled, with the following commands:
  + winrm quickconfig (answer y to both questions)
  + winrm set winrm/config/service/Auth @{Basic="true"}
  + winrm set winrm/config/service @{AllowUnencrypted="true"}
  + winrm set winrm/config/winrs @{MaxMemoryPerShellMB="1024"}
* a fixed administrator password (there's no way for the plugin
to figure out the random password of the base EC2 Windows AMI)

This changeset enhance the Slave Template configuration to specify a
Windows AMI. In which case it is necessary to enter the correct
windows administrator password of the configured remote user.

Note: the current WinRM implementation is not running onto encrypted
transport, nor use encrypted payloads. This will come in a subsequent
patch.

Note2: the current WinRM implementation is roughly based on Xebialabs'
Overthere and WinRB ruby implementation.

This support is considered experimental, even though it has been
successfully tested.

Signed-off-by: Brice Figureau <brice@daysofwonder.com>