Checkstyle added

[bartlomiejgraczyk]

I've applied most of the google's java codestyle rules.

useful tasks:
./gradlew checkstyleMain
./gradlew checkstyleTest

both are run during ./gradlew check

[codecov-commenter]

Codecov Report

Merging #145 (7971737) into master (c368d45) will increase coverage by 0.00%.
The diff coverage is 98.17%.

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@@            Coverage Diff            @@
##             master     #145   +/-   ##
=========================================
  Coverage     98.82%   98.83%           
  Complexity      513      513           
=========================================
  Files            76       76           
  Lines          2050     2058    +8     
  Branches         79       79           
=========================================
+ Hits           2026     2034    +8     
  Misses           12       12           
  Partials         12       12           

Files	Coverage Δ
...pl/allegro/finance/tradukisto/ValueConverters.java	100.00% <100.00%> (ø)
...allegro/finance/tradukisto/internal/Container.java	100.00% <100.00%> (ø)
...o/finance/tradukisto/internal/NumberProcessor.java	100.00% <100.00%> (ø)
.../converters/BigDecimalToBankingMoneyConverter.java	100.00% <100.00%> (ø)
...to/internal/converters/NumberToWordsConverter.java	96.55% <100.00%> (ø)
...es/bulgarian/BulgarianIntegerToWordsConverter.java	100.00% <ø> (ø)
.../internal/languages/bulgarian/BulgarianValues.java	100.00% <ø> (ø)
...to/internal/languages/croatian/CroatianValues.java	100.00% <ø> (ø)
.../languages/czech/CzechIntegerToWordsConverter.java	100.00% <100.00%> (ø)
...adukisto/internal/languages/czech/CzechValues.java	100.00% <ø> (ø)
... and 31 more