Fix memcached extension _format_key error in python3

In python3 memcached key generation for long unicode keys failed, because unicode object is not consumable by sha1 function (beaker/ext/memcached.py in _format_key).
I added check if python version is > 3.0 and encoded unicode formated_key to bytes using utf-8 (will work for ascii too). Added test for long unicode strings. It fails on python >= 3.0 without patch and passes with it.
Also I increased fudge time in test_container.py because tests failed on Travis (slow).