Pyvalid

- Fixed the `IterableValidator` validator
- Refactoring
- Documentation updates
- New unit tests
- Split base/dev requirements
- Flake8 fixes

Changes:
- Breaking change: the `InvalidReturnType` class has been renamed to the `InvalidReturnTypeError`
- Created the `PyvalidError` class — base class for exceptions raised by the pyvalid module
- Fixed the issue with importing the `pyvalid.validators` module
- Refactoring (without affecting functionality, I hope)

- Added the `IterableValidator` validator
- Converted the `validators.py` module into separate python files
- Extended test coverage
- Fixed issues detected by flake8

Updated pypi classifiers

Fixed PY 3.5 related issue

Deps fix