Add server delete to readme
Refactor server creation validation into method
Add support for metadata on server create

- metadata can be supplied as knife[:joyent_metadata]
  in knife.rb, or can be supplied as JSON on the command line
- command line takes precedence over values set in knife.rb
- only a single level of key/values can be set at the moment
Merge pull request #34 from livinginthepast/master

Add support for metadata on server create