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.
Keep in mind that 'constant' and 'final' mean different things.  In google, 'constant' is referring to statements such as

```java
static final int GOOD_STATUS_CODE = 0;
static final int BAD_STATUS_CODE = 1;
```
These statements can not mutate, and are more human friendly names for the given values.  

These statements are very similar to define in c

```c
#define GOOD_STUS_CD 0
#define BAD_STUS_CD 1
```

Instance fields that are final do not imply 'constant' in these same terms.  Both are references are immutable, but their meaning is different.