Add npm script for release versioning

Version is set in `package.json` (used as npm package version) and in
`src/version.js` (used to create driver's user agent string).

Script is named `versionRelease` and requires `VERSION` env variable to be set.
Examples:
```
    VERSION=1.1.5 npm run versionRelease
    VERSION=1.1.5-rc01 npm run versionRelease
    VERSION=1.1.5-alpha03 npm run versionRelease
```

Author: lutovich

package.json

@@ -15,7 +15,8 @@
     "start-neo4j": "gulp start-neo4j",
     "stop-neo4j": "gulp stop-neo4j",
     "run-tck": "gulp run-tck",
-    "docs": "node_modules/.bin/esdoc -c esdoc.json"
+    "docs": "node_modules/.bin/esdoc -c esdoc.json",
+    "versionRelease": "gulp set --version $VERSION && npm version $VERSION --no-git-tag-version"
   },
   "main": "lib/index.js",
   "devDependencies": {