first attempt at suppressing distributed lock release errors

This is an approach to solving https://bitbucket.org/zzzeek/dogpile.cache/issues/77/handling-redis-lock-timeouts

A RedisDistributedLockProxy class is used to wrap the lock and suppress errors on release. It is enabled by sending the argument lock_safe_release = True

The test-suite was amended to test for this. The tests against get_or_create and get_or_create_multi simply sleep for 1 second longer than the lock timeout.