Increase the code coverage and PEPs standards

**Problem**
There are no tests, so developers can't make pull requests without breaking the code. The issue probably related with #40
Also, need to standardize the code as much as possible.
At the moment these PEPs are ignored: `E126,E127,E128,E123,E226,E241,E242,E265,W503,E402`

**Solution**
1. Increase the code coverage as much as possible.
2. Integrate with CI, (i.e. CircleCI, TravisCI). Add a test runner for the each pull request.
3. Step by step make a code refactoring and standardize simultaneously, later add a linter for PRs.
4. Add/Clarify a documentation how to contribute.