Add to Zeppelin coding convention

This PR is about adding to zeppelin some standard in code level. 
The goal of this Pr is to improve the readability of Zeppelin code (right now it is a mess and it is hard to read).

A checkstyle plugin is added to check the code according to a set of rules defined in `_tools/checkstyle.xml`, this rules are from the Google style convention for [Java style](http://google-styleguide.googlecode.com/svn/trunk/javaguide.html)

The checkstyle plugin will create a report under `{zeppelin_subproject}/target/site/checkstyle.html`