switch to official AWS Go SDK

This uses the official Go SDK from Amazon, which supports the newer v4
authentication method.  Fixes #74.  Doing so also required adding a new
s3cache package which uses the official SDK.

THIS IS A BREAKING CHANGE for anyone that uses s3, since the syntax of
the command line flag is now different.  This is unfortunately necessary
because aws-sdk-go always requires the region to be explicitly declared,
which wasn't always the case with the previous format.  This breaking
change is unfortunate, but given that the other s3 package hasn't seen
updates in years, and so many new S3 regions only support the newer v4
authentication method, it's necessary.

README.md

@@ -98,10 +98,12 @@ enabled using the `-cache` flag.  It supports the following values:
    available memory and is not recommended for production systems)
  - directory on local disk (e.g. `/tmp/imageproxy`) - will cache images
    on disk
- - s3 URL (e.g. `s3://s3-us-west-2.amazonaws.com/my-bucket`) - will cache
+ - s3 URL (e.g. `s3://region/bucket-name/optional-path-prefix`) - will cache
    images on Amazon S3.  This requires either an IAM role and instance profile
    with access to your your bucket or `AWS_ACCESS_KEY_ID` and `AWS_SECRET_KEY`
-   environmental variables be set.
+   environmental variables be set. (Additional methods of loading credentials
+   are documented in the [aws-sdk-go session
+   package](https://docs.aws.amazon.com/sdk-for-go/api/aws/session/)).
  - gcs URL (e.g. `gcs://bucket-name/optional-path-prefix`) - will cache images
    on Google Cloud Storage.  This requires `GCP_PRIVATE_KEY` environmental
    variable be set.