Bump version to 1.5.0

Also add 'benchmark' command

Author: qwertie

package.json

@@ -1,6 +1,6 @@
 {
   "name": "sorted-btree",
-  "version": "1.4.0",
+  "version": "1.5.0",
   "description": "A sorted list of key-value pairs in a fast, typed in-memory B+ tree with a powerful API.",
   "main": "b+tree.js",
   "typings": "b+tree",
@@ -9,7 +9,8 @@
     "build": "tsc && npm run minify",
     "minify": "uglifyjs -cm -o b+tree.min.js -- b+tree.js",
     "prepare": "npm run build",
-    "safePublish": "npm run build && testpack && npm publish"
+    "safePublish": "npm run build && testpack && npm publish",
+    "benchmark": "npm run build && node benchmarks.js"
   },
   "files": [
     "b+tree.js",