TACHYON-182 Add checkstyle to the maven build system so we could reduce time to do style check for contributions

TACHYON-182 Add checkstyle to the maven build system so we could reduce time to do style check for contributions following Google style.
Fixed violations reported in the current master branch.

Note that currently you need to run
  mvn checkstyle:check or mvn checkstyle:checkstyle
to run the check.

You will see some warnings on public member for Tachyon configuration classes:
  CommonConf
  MasterConf
  WorkerConf
  UserConf
due to naming of their public member as all caps.

Once we stabilize the code we could hook it to build system to fail when checkstyle fail.
It is copied from checkstyle github page. The XML file is to be manually
generated following the schema and doc from checkstyle website.
The team provides sample checkstyle XML config that follows Google Java
style guide which we are using here.

On Saturday, October 18, 2014, Haoyuan Li <notifications@github.com> wrote:

> In core/src/main/resources/tachyon_checks.xml:
>
> > +
> > +    Checkstyle configurartion that checks the Google coding conventions from:
> > +
> > +    -  Google Java Style
> > +       https://google-styleguide.googlecode.com/svn-history/r130/trunk/javaguide.html
> > +
> > +    Checkstyle is very configurable. Be sure to read the documentation at
> > +    http://checkstyle.sf.net (or in your downloaded distribution).
> > +
> > +    Most Checks are configurable, be sure to consult the documentation.
> > +
> > +    To completely disable a check, just comment it out or delete it from the file.
> > +
> > +    Copied from https://github.com/checkstyle/checkstyle/blob/master/google_checks.xml
> > +
> > + -->
>
> How is this file generated?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/amplab/tachyon/pull/473/files#r19057031>.
>