Comulative fixes and improvements

* Russian translation, more translatable strings in templates
* Delete event view now works
* Fix for Period model and it's descendants when USE_TZ project's setting is set to False
* Fixes for tests and code style improvements

I'm using my own fork of this great app in one of my commercial projects. I'm trying to fix as more bugs as possible and return results as single comulative PR to your fork. Let me know is this model is correct in your opinion? Or I should divide this PR in many small PRs? Or you don't need PRs at all :smile: