Add semver middleware

This uses the http-proxy module to send requests to another API instance
based on the version string contained in the request.

middleware/semver.js

@@ -0,0 +1,25 @@
+var httpProxy = require('http-proxy');
+
+var proxy = new httpProxy.createProxyServer();
+
+function setup(peliasConfig) {
+  var currentVersion = peliasConfig.version; //TODO :add to pelias-config
+
+  var middleware = function middleware(req, res, next){
+    var reqVersion = req.params.vr;
+    // if URI contains v1, handle locally
+    if (currentVersion === reqVersion) { // uri contains v1
+      next();
+    } else if (peliasConfig.proxyMap.hasOwnProperty(reqVersion)){ // else send to legacy server
+      var host = peliasConfig.proxyMap[reqVersion];
+      req.url = req.url.substring(1 + reqVersion.length);
+      proxy.proxyRequest(req, res, { target: host });
+    } else {
+      throw new Error('can\'t handle this version');
+    }
+  };
+
+  return middleware;
+}
+
+module.exports = setup;

package.json

@@ -41,6 +41,7 @@
     "geojson": "^0.2.1",
     "geojson-extent": "^0.3.1",
     "geopipes-elasticsearch-backend": "^0.2.0",
+    "http-proxy": "^1.11.1",
     "is-object": "^1.0.1",
     "markdown": "0.5.0",
     "microtime": "1.4.0",