Adding bucket_as_host flag to generate signed url

[gr8Adakron]

The following PR will allow us to create signed url in the following nomenclature: https://custom.domain.com/image.jpg? which wasn't feasible before with this library. This allow us to be independent of amazon host. As in real time scenario when generating signed url for clients side application the IP whitelisting is a well known problem. Certainly this resolve the prior issue as one get better control over custom domain IPs.
Further benefits with been custom domain as host - provision us to configure authentication layer on access of presigned url too.

Inspired from the following issues:

• getSignedUrl() - Custom domain name? aws/aws-sdk-js#891
• Create Signed URL with custom domain #154

[gr8Adakron]

@bernardd FYI

[bernardd]

Hi @gr8Adakron - the change itself looks good; just a couple of little requests:

• I think the s3_bucket_endpoint option might be better named something like bucket_as_host (unless AWS use the former nomenclature somewhere).
• Could you add the new option, whatever it ends up being called, to the presigned_url_opts() type spec
• Could you also update the docs for presigned_url() to reflect the new option (I just noticed the existing docs for virtual_host are slightly wrong, too, which is probably what caused the confusion over in Create Signed URL with custom domain #154).

Thanks!

[gr8Adakron]

@bernardd I choose this s3_bucket_endpoint argument as it was already been considered in other libraries.

• I was thinking to add separate PR for documentation update - to keep things simple.

[bernardd]

Also it would be great to have a test case to cover this use.

[gr8Adakron]

@bernardd

• Renamed flag to bucket_as_host - as its easy to understand.
• Adding REAME.md for bucket_as_host flag
• Adding code level doc for bucket_as_host flag
• Fixing code level doc for virtual_host flag
• Adding test cases for bucket_as_host flag in coupled with virtual_host flag.

Please review.

[bernardd]

Thanks @gr8Adakron - that all looks good. One tiny little request. The docs say:

When option param :bucket_as_host is true, the bucket name will be used as the hostname, which will look like <bucket>.<domain>.com host.

but actually bucket is the full hostname, there's no additional domain or .com parts. Could you reword it to something like

When option param :bucket_as_host is true, the bucket name will be used as the full hostname. In this case, bucket must be set to a full hostname, for example mybucket.example.com.

If you can make that tweak, I'll get this all merged. Thanks for all your work on it :)

[gr8Adakron]

@bernardd Updated for the mentioned tweak.

[bernardd]

Fantastic, thanks heaps for your work on this @gr8Adakron!