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.
We should add public method accessors instead of use public member
variables.
But that could go in different PR I suppose.

On Saturday, October 18, 2014, Haoyuan Li <notifications@github.com> wrote:

> In core/src/main/java/tachyon/client/TachyonByteBuffer.java:
>
> > @@ -9,7 +9,7 @@
> >   */
> >  public class TachyonByteBuffer implements Closeable {
> >    // ByteBuffer contains data.
> > -  public final ByteBuffer DATA;
> > +  public final ByteBuffer mData;
>
> Why not DATA?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/amplab/tachyon/pull/473/files#r19057748>.
>