Add example unit test and config for testing

Add testing configuration:
 - pytest as test framework
 - tox for creating testing environments and running tests (run with `tox`)
 - pytest-asycio for asyncio testing
 - coverage/pytest-cov for measuring test coverage
 - flake8 for pep8 checking

I've also added one quick example test demonstrating basic unit testing
and use of the basic fixtures and marks provided by pytest-asyncio.

Just this already covers a suprising 32% of the codebase, mostly through
imports, but I wouldn't expect it to be helpful yet. This should provide
a good base to build on in the future though.

tox.ini

@@ -0,0 +1,22 @@
+[tox]
+envlist = py35,py36,py37,py38
+
+[testenv]
+deps =
+    -rrequirements.txt
+    -roptional-requirements.txt
+    -rdev-requirements.txt
+commands =
+    # NOTE: you can run any command line tool here - not just tests
+    pytest {posargs}
+
+# run with tox -e flake
+[testenv:flake]
+deps =
+    -rrequirements.txt
+    -roptional-requirements.txt
+    -rdev-requirements.txt
+    flake8
+commands =
+# TODO: move options to dedicated flake8 config
+    flake8 --exclude telethon/tl/,telethon/errors/rpcerrorlist.py --ignore E501,F401 telethon/ tests/