Add checkstyle plugin

[codecov[bot]]

Codecov Report

Merging #67 (9ead21f) into master (42e5260) will increase coverage by 0.05%. The diff coverage is 86.36%.

@@             Coverage Diff              @@
##             master      #67      +/-   ##
============================================
+ Coverage     84.78%   84.84%   +0.05%     
- Complexity      767      769       +2     
============================================
  Files            60       60              
  Lines          2209     2210       +1     
  Branches        325      325              
============================================
+ Hits           1873     1875       +2     
- Misses          191      192       +1     
+ Partials        145      143       -2     

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/baidu/hugegraph/date/SafeDateFormat.java	100.00% <ø> (+7.69%)	6.00 <0.00> (ø)
...ain/java/com/baidu/hugegraph/util/CheckSocket.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...ain/java/com/baidu/hugegraph/util/VersionUtil.java	59.45% <33.33%> (-1.66%)	9.00 <0.00> (ø)
...main/java/com/baidu/hugegraph/rest/RestResult.java	86.66% <87.50%> (ø)	9.00 <5.00> (ø)
...n/java/com/baidu/hugegraph/config/OptionSpace.java	91.30% <100.00%> (ø)	11.00 <5.00> (ø)
...in/java/com/baidu/hugegraph/testutil/Whitebox.java	89.61% <100.00%> (ø)	22.00 <0.00> (ø)
...ain/java/com/baidu/hugegraph/util/NumericUtil.java	83.59% <100.00%> (ø)	68.00 <0.00> (ø)
...n/java/com/baidu/hugegraph/concurrent/KeyLock.java	80.39% <0.00%> (+1.96%)	26.00% <0.00%> (+1.00%)
.../main/java/com/baidu/hugegraph/perf/Stopwatch.java	87.75% <0.00%> (+2.04%)	21.00% <0.00%> (+1.00%)

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