Make ServerDown mapping is now conditional

Refs #66
Add variable initialization that was missed in 00575f14f9
Merge pull request #67 from ottob/master

00575f14f9 broke get_multi
Add retry_timeout to behaviors test
Fix doctests for exception strings
Undo misguided changes in 00575f14f9d8521d259630abd18b
Clean up incr_multi implementation
Fix reference counting in get_multi
Merge pull request #69 from peekeri/get_multi_decref

Fix reference counting in get_multi
Improve background for docs
Portability: Skip strndup in favor of strdup

Thanks to Marcel Bankmann for reporting and early testing. Closes #70.
Fix typo
Fix autocorrection of old _retry_timeout behavior name

Thanks to @jhansche for spotting.
Test for signedness overflow in incr/decr

Fixes #74
Test for negative deltas in incr/decr

Fixes #73
Nitpick header format
Fix missing closing bracket
Adding ability to support dead_timeout

MEMCACHED_BEHAVIOR_DEAD_TIMEOUT for libmemcached>=1.0.3 allows you to retry dead
servers correctly--the new code explicitly checks to make sure that this value
is set when trying to reconnect dead servers, and typically needs to be set
alongside retry_timeout to get any real fail over recovery.
Adding documentation for dead_timeout
Updating doctests to reflect addition of dead_timeout