Add implementation for email_validation.validate_email()

## the math
* [ ] http://stackoverflow.com/questions/3371255/writing-unit-tests-in-python-how-do-i-start
* [ ] https://jeffknupp.com/blog/2013/12/09/improve-your-python-understanding-unit-testing/
* [ ] http://docs.python-guide.org/en/latest/writing/tests/
* [ ] https://cgoldberg.github.io/python-unittest-tutorial/
* [ ] https://habrahabr.ru/post/121162/
* [ ] https://pythonworld.ru/moduli/modul-unittest.html

## the practice
* [x] Branch: `feature/validate-email-implementation`
* [x] Add test cases to `test_email_validation.py`
* Use `assert*` methods from [this section](https://docs.python.org/3.6/library/unittest.html#test-cases)
* Write 'positive tests' (with valid emails passed in different formats)
* Write 'negative tests' (with different forms of invalid emails passed)
* Feel free to create a PR at this point, not waiting for the last instruction
* [ ] **After that** add implementation detail to `email_validation.py` and make sure tests pass
* 1. run tests
* 2. if tests succeed go to (v)
* 3. if tests fail fix the `validate_email` function
* 4. go to (i)
* 5. complete!
* [ ] Finally make a PR with this branch