Feature/aws sdk v2

[monwolf]

should address #555 (comment)

[alekc]

@monwolf I have added some additions to your pr (see the link above). On top of my head there were following issues

• SDKv2 need to have support for paths enabled to behave in the same way as with sdk1 (with v2 it uses domain as a primary bucket identification). Introduced AWS_S3_USE_PATH_STYLE to disable this behaviour
• with sdkv2, custom endpoint now requires explicit protocol definition (added a proper error warning in case its missing)
• ACL are passed only if they are defined

Reason for simply not adding a review is that I needed a working version for my environment.