Allow custom S3 domain

[bbailleux]

This PR aims at allowing user to specify an alternate domain as request target. Thus, it will be possible to use a personal or custom instance of S3-compatible on-line storage service.

Especially useful for companies operating private cloud with S3-compatible service.

Two steps:

First step: lint

The first commit is:

• Apply changes required to run successfully shellcheck on all scripts (see style guide: https://google.github.io/styleguide/shellguide.html#shellcheck)
• Fix a few style guide oversights
• Fix an incomplete test in assertArgument()
• Update README to comply with linter issues
• Update URL of style guide (as googlecode.com no longer exists)

Second step: new feature

The new feature has been proposed in second commit

It adds a -d or --domain command-line option (or make use of S3_DEFAULT_DOMAIN env. var.) to replace default domain (i.e. s3.amazonaws.com) with custom domain

[bbailleux]

Hello @cvle

Can you please consider either reviewing this PR or archiving this repo, so that I can apply the proposed change to my own fork of this nice and useful tool?

Thanks in advance / Danke schön.