Add PMD as part of the build process

[NewestUser]

I would like to have PMD integrated into the build process.

This would forbid merging code that does not comply with common guidelines. Example: if it contains unused variables/ imports or etc..

Here is the setup guide.

NOTE: travis should fail the build if the maven PMD check does not pass.

[NewestUser]

Adding @IvoStaynov @rositsabodurova for visibility

[dahmedov]

The task is done.