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.
For example when you declare:
  ByteBuffer buf = .....;
and then after 5 lines of code later the first use of that buf variable happen. The distance limit is the number of lines between a variable is declared to its first usage, unless it is declared as final.
This follows Googls style 4.8.2.2 https://google-styleguide.googlecode.com/svn/trunk/javaguide.html#s4.8.2-variable-declarations.