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?
Some of the resource specs should be updated. I pushed a commit this morning that removes the need to provide a region and credentials to a client under test.  Instead you should be able to construct your `EC2::Client` with `stub_response: true`. This prevents the client from making actual HTTP requests and it also provides default `:region` and `:credentials` configuration values.