Add REST backend

This is a port of the original work by @bchapuis in
https://github.com/restic/restic/pull/253

doc/REST_backend.md

@@ -0,0 +1,56 @@
+REST Backend
+============
+
+Restic can interact with HTTP Backend that respects the following REST API.
+
+## HEAD /config
+
+Returns "200 OK" if the repository has a configuration,
+an HTTP error otherwise.
+
+## GET /config
+
+Returns the content of the configuration file if the repository has a configuration,
+an HTTP error otherwise.
+
+Response format: binary/octet-stream
+
+## POST /config
+
+Returns "200 OK" if the configuration of the request body has been saved,
+an HTTP error otherwise.
+
+## GET /{type}/
+
+Returns a JSON array containing the names of all the blobs stored for a given type.
+
+Response format: JSON
+
+## HEAD /{type}/{name}
+
+Returns "200 OK" if the blob with the given name and type is stored in the repository,
+"404 not found" otherwise. If the blob exists, the HTTP header `Content-Length`
+is set to the file size.
+
+## GET /{type}/{name}
+
+Returns the content of the blob with the given name and type if it is stored in the repository,
+"404 not found" otherwise.
+
+If the request specifies a partial read with a Range header field,
+then the status code of the response is 206 instead of 200
+and the response only contains the specified range.
+
+Response format: binary/octet-stream
+
+## POST /{type}/{name}
+
+Saves the content of the request body as a blob with the given name and type,
+an HTTP error otherwise.
+
+Request format: binary/octet-stream
+
+## DELETE /{type}/{name}
+
+Returns "200 OK" if the blob with the given name and type has been deleted from the repository,
+an HTTP error otherwise.