S3 patch

[qianl15]

(Sorry I screwed up the last pull request, so I reopened a new one.)
Try to add S3 storage support for Scanner. Also need to update storehouse package.

In order to use S3, the user needs to provide aws_access_key_id and aws_screte_access_key:

export AWS_ACCESS_KEY_ID=<Access Key>
export AWS_SECRET_ACCESS_KEY=<Secret>

And a demo of ~/.scanner.toml file to use S3:

[storage]
type = "s3"
bucket = "<your bucket name>"
db_path = "scanner_db"
region = "us-west-2(change to your S3 bucket region)"
endpoint = "s3.dualstack.us-west-2.amazonaws.com (change to your S3 endpoint)"

[fpoms]

@qinglan233: can you rebase onto the most recent master? We've pushed a fix that should fix the travis build failing for PRs.

[qianl15]

The travis build returns:

Error response from daemon: No such image: scannerresearch/scanner:cpu

Have no idea...

[fpoms]

@qinglan233: sorry, this should be fixed now. Please try rebasing again!

[fpoms]

@qinglan233: Ugh, sorry, working with Travis can be a pain sometimes... trying a fix for this new issue now.

[fpoms]

Closes #87