SNS doesn't work on AppEngine

[andrewwatson]

Because you can't use http.DefaultClient, you can't use http.Get()

I started working on some changes to allow you to create an http.Client and pass it to the SNS to use but now i'm getting 400 errors from AWS so I'm trying to figure out what's wrong.

[rafb43]

You might be running into either of these 2 problems:

• sns.sign uses SignatureVersion=2 which is not supported by SNS. You need to use version 4. https://github.com/smartystreets/go-aws-auth has this implemented correctly but doesn't play too well with goamz as is.
• TargetArn is not passe down to the request as TopicArn is, and then you get a 400. This is easy to fix