S4cmd

- Added `--endpoint_url` flag to allow s4cmd to work with non-s3 object storage services ([82](https://github.com/bloomreach/s4cmd/pull/82))
- Fix bug in pip install ([102](https://github.com/bloomreach/s4cmd/pull/102))
- Fix bug which was leading to errors on zero length files ([81](https://github.com/bloomreach/s4cmd/pull/81))
- Add flag `--version` to display s4cmd version
- Check added to ensure consistency of `os.write` in method `write_file_chunk`
- Full E2E test-suite running on python 2 and 3, backed by Travis-CI

- Merge change from rameshrajagopal for S3 keys in command-line parameters.

- Fully migrated from old boto 2.x to new boto3 library.
- Support S3 pass through APIs.
- Support batch delete (with delete_objects API).
- Support S4CMD_OPTS environment variable.
- Support moving files larger than 5GB with multipart upload.
- Support timestamp filtering with --last-modified-before and --last-modified-after options.
- Faster upload with lazy evaluation of md5 hash.
- Listing large number of files with S3 pagination, with memory is the limit.
- New directory to directory dsync command to replace old sync command.

- Add bash command line completion

- Add compatibility for Python3

- Merge changes from linsomniacgithub for better argument parsing