* Bump v3.0.0

* Adding Slack invitation link

Introduce `Exploit.validate_setup` method in order to check whether
Exploit's setup is correct. In most case scanarios we will check
if `Exploit.target` is not `None`. When exploit need custom
validation logic please overwrite `validate_setup`.

LGTM

* Move tests to tests/ directory
* Remove unused `tox.ini` file
* Add flake8 compliance
* Add `lint` target to `tests` target in the Makefile