[TACHYON-199] Enable checkstyle:check on mvn install.

Enable checkstyle:check on mvn install to help contributors following code style guidelines.

When code style is violated the mvn install will barf with error:

INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] 
[INFO] Tachyon Project Parent ............................ SUCCESS [  1.184 s]
[INFO] Tachyon Project Core .............................. FAILURE [ 15.091 s]
[INFO] Tachyon Project Client ............................ SKIPPED
[INFO] Tachyon Project Assemblies ........................ SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 16.474 s
[INFO] Finished at: 2014-10-28T13:36:19-08:00
[INFO] Final Memory: 14M/246M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-checkstyle-plugin:2.13:check (checkstyle) on project tachyon: Failed during checkstyle execution: There are 1 checkstyle errors. -> [Help 1]

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