Add setup, fix bugs, introduce acl.AbstractACLGroupsCallback and new autz middleware, add pytests.

[ilex]

What do these changes do

• Add setup functions for auth and acl middleware to install them in aiohttp fashion.
• Add pytests: as the original tests don't run with current aiohttp version and require utils to
  be rewritten, I use a pytest lib with pytest-aiohttp plugin and unlike original tests
  with real aiohttp.Application. All tests were put into separate folder pytests:
  • All original tests were added with pytest.
  • Add some new tests to test untested functionality in auth and acl.
  • Add tests for all new functions, classes and modules.
• Fix bugs:
  • The bug in acl_required decorator mentioned in acl_required decorator crash #1.
  • A possible security issue with acl groups. The issue is follow: the default behavior is
    to add user_id to groups for authenticated users by the acl middleware, but if
    user_id is equal to some of acl groups that user suddenly has the permissions he is not
    allowed for. So to avoid this kind of issue user_id is not added to groups any more. A test
    to reproduce the issue is also added.
• Introduce AbstractACLGroupsCallback class in acl middleware to make it possible easily create
  callable object by inheriting from the abstract class and implementing acl_groups method. It
  can be useful to store additional information (such database connection etc.) within such class.
  An instance of this subclass can be used in place of acl_groups_callback parameter.
• Introduce generic authorization middleware autz that performs authorization through the same
  interface (autz.permit coroutine and autz_required decorator) but using different policies.
  Middleware has the acl authorization as the built in policy which works in the same way as acl
  middleware. Users are free to add their own custom policies or to modify ACL one.
• Add global aiohttp_auth.setup function to install auth and autz middlewares at once
  in aiohttp fashion.
• Rewrite documentation in README.rst to reflect all changes. Introduce new modules and classes
  and usage examples in docs.

Known issues

There is one issue with auth middleware. In class TktAuthentication documentation says about
parameter reissue_time: If this value is 0, a new ticket will be reissued on every request which requires authentication.. But it seems that if two requests are handled during one second
the value of ticket would be the same. So tests which test the reissue_time should wait a second
between requests to retrieve an actually new value.