Added create-ami command

(Mitchell, as discussed I'm moving the conversation to GitHub)

This pull request adds a "create-ami" command to the vagrant-aws plugin. As might be guessed from the name it is responsible for creating Amazon Machine Images based on EC2 instances that have been provisioned with Vagrant.

Use case: My current workflow for creating AMIs is to launch the instance with Vagrant, provision and test it, and then create an AMI via a Python script. Adding the create-ami command means I can move all of this workflow to Vagrant, simplifying the process.

Created AMIs can be assigned tags based on information in the Vagrantfile.