AWS signature V4 support (eu-central-1 and other new buckets)

This pull request (for commit 95f8cab13918dc7767394b6ea47fb979f4da79b1) adds support for AWS V4 version.
Example:
$ s3fs BUCKET MOUNTPOINT -o endpoint=eu-central-1,url=https://s3.eu-central-1.amazonaws.com
You can checkout it here https://github.com/caxapniy/s3fs-fuse/tree/1.77v4merge

I have modified version https://github.com/shunataws/sv4s3fs
Changes:
- removed hardcoded China S3 servers, added command-line argument "endpoint"
- added content-md5 checking
- added multipart upload support

I was not able to merge branches - there are conflicts(caused by changes in HEAD)
Hope this request helps someone to merge changes to HEAD.