Add Bruch.io to mvn build process for zeppein-web

First attempt was to use [brunch-maven-plugin](https://github.com/mulesoft/brunch-maven-plugin) but it failed (see SHA: 832d53a344162d6b5da54bfcbbb4acb7a257c37d)
Current one just use maven exec plugin, assuming dependencies from https://github.com/NFLabs/zeppelin/tree/master/zeppelin-web#build are already installed 

@Leemoonsoo Does that look good enough? This should work now. Can you verify?
```
mvn assemble:assemble
./bin/zeppein-demon.sh start
```