Add checkstyle for enforcing coding conventions

[SKrPl]

• Add checkstyle (preferably google's)
• Add checkstyle checking at the build time

[abhijeetviswa]

Draft PR #217 aims to fix existing lint errors. It will also add in a CI job to run for every PR thus ensuring the code base doesn't start to smell. Once that PR is merged in, we'll add Checkstyle.