Add XenApi for Xen/XenServer

Add support for XenAPI via xenapi-ruby gem.
Initial methods scaffolded from VMWare vSphere and Fog

Tested against XenServer6.1 and 6.0.2

Testing procedure:
```
controller = ::Lab::Controllers::VmController.new
controller.build_from_running(opts)
controller[<name from name_label field of a running VM>]
```

TODO: OpenSSL still refuses to accept self-signed certificates.
Would also be nice to rewire XMLRPC::Client to run over Rex::Socket
This may also help to maintain session keepalives and query many
endpoints conrurrently for faster processing.
XenAPI use SSL - resolves self signed cert bug

Ruby's XMLRPC interface does not like self signed certificates.
XMLRPC sessions can be monkeypatched on the fly to ignore SSL
certificates before the initial connection attempt is made. With
XenAPI, the session is created and connected in the same method.
This approach resolves XenAPI upstream SSL validation as well.
Merge pull request #4 from sempervictus/xenapi_driver

Add XenApi for Xen/XenServer