Implement S3 Gateway to third party cloud storage providers. (#3756)

Currently supported backend is Azure Blob Storage.

```
export MINIO_ACCESS_KEY=azureaccountname
export MINIO_SECRET_KEY=azureaccountkey
minio gateway azure
```

docs/gateway/README.md

@@ -0,0 +1,48 @@
+# Minio Gateway [![Slack](https://slack.minio.io/slack?type=svg)](https://slack.minio.io)
+
+Minio gateway adds Amazon S3 compatibility to third party cloud storage providers. Supported providers are:
+
+- Azure Blob Storage
+
+## Run Minio Gateway for Azure Blob Storage
+
+### Using Docker
+
+```
+docker run -p 9000:9000 --name azure-s3 \
+ -e "MINIO_ACCESS_KEY=azureaccountname" \
+ -e "MINIO_SECRET_KEY=azureaccountkey" \
+ minio/minio gateway azure
+```
+
+### Using Binary
+
+```
+export MINIO_ACCESS_KEY=azureaccountname
+export MINIO_SECRET_KEY=azureaccountkey
+minio gateway azure
+```
+
+## Test using Minio Client `mc`
+`mc` provides a modern alternative to UNIX commands such as ls, cat, cp, mirror, diff etc. It supports filesystems and Amazon S3 compatible cloud storage services.
+
+### Configure `mc`
+
+```
+mc config host add myazure http://gateway-ip:9000 azureaccountname azureaccountkey
+```
+
+### List containers on Azure
+
+```
+mc ls myazure
+[2017-02-22 01:50:43 PST]     0B ferenginar/
+[2017-02-26 21:43:51 PST]     0B my-container/
+[2017-02-26 22:10:11 PST]     0B test-container1/
+```
+
+## Explore Further
+- [`mc` command-line interface](https://docs.minio.io/docs/minio-client-quickstart-guide)
+- [`aws` command-line interface](https://docs.minio.io/docs/aws-cli-with-minio)
+- [`minfs` filesystem interface](http://docs.minio.io/docs/minfs-quickstart-guide)
+- [`minio-go` Go SDK](https://docs.minio.io/docs/golang-client-quickstart-guide)

docs/gateway/azure-limitations.md

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+## Minio Azure Gateway Limitations
+
+Gateway inherits the following Azure limitations:
+
+- Maximum Multipart part size is 100MB.
+- Maximum Multipart object size is 10000*100 MB = 1TB
+- No support for prefix based bucket policies. Only top level bucket policy is supported.
+- Gateway restart implies all the ongoing multipart uploads must be restarted.
+  i.e clients must again start with NewMultipartUpload
+  This is because S3 clients send metadata in NewMultipartUpload but Azure expects metadata to
+  be set during CompleteMultipartUpload (PutBlockList in Azure terminology). We store the metadata
+  sent by the client during NewMultipartUpload in memory so that it can be set on Azure later during
+  CompleteMultipartUpload. When the gateway is restarted this information is lost.
+- Bucket names with "." in the bucket name is not supported.
+- Non-empty buckets get removed on a DeleteBucket() call.
+
+Other limitations:
+- Current implementation of ListMultipartUploads is incomplete. Right now it returns if the object with name "prefix" has any uploaded parts.
+- Bucket notification not supported.