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 was due to distance limit between variable declaration and it's first
usage.
In java it is recommended to only declare variable near to first usage
unless it is tagged as final.
Declaring all scope variables at top of method start is C style.
But it is always good practice in modern Java to declare local variables to
final when it is not expected to change to promote immutability.

On Saturday, October 18, 2014, Haoyuan Li <notifications@github.com> wrote:

> In core/src/main/java/tachyon/client/TachyonFile.java:
>
> > @@ -405,7 +405,7 @@ private TachyonByteBuffer readLocalByteBuffer(int blockIndex, long offset, long
> >          }
> >
> >          FileChannel localFileChannel = localFile.getChannel();
> > -        ByteBuffer buf = localFileChannel.map(FileChannel.MapMode.READ_ONLY, offset, len);
> > +        final ByteBuffer buf = localFileChannel.map(FileChannel.MapMode.READ_ONLY, offset, len);
>
> buf is a local variable. We didn't enforce local variables to be final.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/amplab/tachyon/pull/473/files#r19057750>.
>