Cleanup repo and add .gitignore file

[KarolAntczak]

Hi,

First of all - very nice project. Congratulations!

This repo contains some stuff, that should not be included into the repo, e.g. out/ directory.
To easily manage what should be included and what not, it is a common practice to add .gitignore file .

• More info about .gitignore files
• Collection of popular .gitignore files