Add Aws::EC2::Instance with support for decrypting windows password

This is incomplete, but attempts to fix #122 with the best sort of documentation; none.

Note: not sure what those inline credentials provide, so making no attempt to use them to create resources to test against - please give me a steer?
Yes, you should be able to safely stub the `#get_password_data` response using `:stub_responses => true`.   I'll give an example in the comments below.