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.

.coveragerc

@@ -0,0 +1,8 @@
+[run]
+branch = true
+parallel = true
+source =
+    telethon
+
+[report]
+precision = 2