wait and user data

replaces pull request https://github.com/airbnb/stemcell/pull/30

- adds wait option so that we can choose to not wait. keeps the current default behavior to do a wait to get the ip address and display date.
- allows user_data to be passed as a parameter

Pretty basic really. This PR is for ec2admin usage.

@igor47 @martin45 @nelgau @pcarrier @daveaugustine @salbito @jeffyip @jtai