huge update:

introduced meta tasks which are used by `_map()`;
introduced task groups;
ending with `gather_and_close()` now;
pool unittests rewritten accordingly;
two new helper classes

setup.cfg

@@ -1,6 +1,6 @@
 [metadata]
 name = asyncio-taskpool
-version = 0.3.5
+version = 0.4.0
 author = Daniil Fajnberg
 author_email = mail@daniil.fajnberg.de
 description = Dynamically manage pools of asyncio tasks