Add custom log formatter, remove slf4j dependency

[sedadas]

Second part of #740

[afdia]

Looks good thanks!

One note for future commits: I'm not saying that it's bad, but to spare you some time which could be better invested in other stuff: You don't have to write javadoc for every method (especially since Umlet is not a library which others use without viewing the Umlet code itself nor will there be external implementations of our interfaces which would benefit from exact javadoc specs).

In general I would suggest to limit comments (or javadoc) to code which is not intuitively understood by a typical Java dev.

Or let's put it that way: put yourself in the shoes of a developer who doesn't know your new code and use comments to increase the understandability of your code. E.g. for background knowledge like "//approach x chosen, because y would have problem z" or for stuff which is not immediately obvious (to think about a concrete example in your commit: "//using Date instead of LocalDateTime because logger should work in GWT which only suports Date")

It will spare you some time and we will still have the most important additional information which is not obvious in the code. Of course you could also add comments to "old Umlet code" which you had problems to understand if you think it will help other developers :)