Update the defaults for droplet creation

DigitalOcean seems to have changed the ID's of their server
images...this update continues to use:

Ubuntu 12.04 x64 Server (id: 284203, distro: Ubuntu)

As the default image.

I also lowered the default droplet size to 512mb.
Merge pull request #39 from pearkes/f-new-creation-defaults

Update the defaults for droplet creation