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

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?
Merge branch 'decrypt_ec2_windows_password' of https://github.com/petemounce/aws-sdk-core-ruby into petemounce-decrypt_ec2_windows_password
Moved the definition of Aws::EC2::Instance#decrypted_window_password

Moved the definition from aws-sdk-core gem to aws-sdk-resources gem.
Also renamed the method to from a noun to a verb, now
`#decrypt_window_password`.
Added changelog entry for few feature.