vm snapshot capability

Hey @astratto, 

The API documentation is very obscure about this, but you can actually snapshot a VM.  It's just unintuitive because your request is sent to the ./vapp/  URI.    You do a ./vapp/***vm***-{id}/action.....   for vm's and ./vapp/***vapp***-{id} for vapp's.  

Because of this behaviour, I pulled the snapshot behaviour out of vapp,  and into connection.rb.    I kept the old method, but the old vapp method is now commented as deprecated,  and delegates to the connection.rb.  

vapp/create_snapshot(id)  -->   connection/create_vapp_snapshot(id, :vapp)
vapp/create_vapp_snapshot(id)  -->   connection/create_vapp_snapshot(id, :vapp)
vm/create_vm_snapshot(id)  -->   connection/create_vm_snapshot(id, :vm)

As you can see I used the same pattern you had for the power_actions common method. 

I want to include mocha,  and do an assertion unit test for when create_snapshot_action (couldn't come up with a better name) is called that the :type  property is being used properly.   So I can put that in before this PR is merged. 

Please comment on implementation - if you'd rather see it implemented a different way - I'd be interested to hear! 

Thanks!