Prompt for engine

[jfhbrook]

This PR makes jitsu require an "engines" field and prompt for it:

For example, here's what happens when you try to deploy jitsu:

% jitsu deploy
info:   Welcome to Nodejitsu
info:   It worked if it ends with Nodejitsu ok
info:   Executing command deploy
warn:   
warn:   Your package.json file is missing required fields:
warn:   
warn:     subdomain,   scripts.start,   engines
warn:   
warn:   Prompting user for required fields.
warn:   Press ^C at any time to quit.
warn:   
help:   
help:   The subdomain is where your application will reside.
help:   Your application will then become accessible at: http://yourdomain.nodejitsu.com
help:   
prompt: subdomain (jitsu): testsubdomain
prompt: scripts.start: ./bin/jitsu
prompt: engines (v0.6.x): 

The engines default is based on your installed version of node.