Contacts assignment

Solution provided based on ZADANI.txt specification.

Major TODOs

• Be prepared for concurrency and race condition and try to search for a better race condition solution.
• Provide jUnit tests with sample http requests.
• To discuss with client reasons whether they really want to use HTTP GET request instead of HTTP POST.
• Fix eventual FIXME in code.
• Describe briefly dataflow and solution architecture here into readme.

What are next possible steps

• Use proper logging library and configuration.
• Improved exception handling.
• Dependencies injection via Spring.