Merge pull request #126 from joe1chen/fix-memcached-rails-increment-decrement

Fix Memcached::Rails increment and decrement to match ActiveSupport's memcache implementation.
Fix Memcached::Rails write method to match ActiveSupport memcache write behavior:  1) support :unless_exist, and 2) convert values to string if :raw is true.
Merge remote-tracking branch 'origin/memcached-rails-fix-write-unless-exist' into integration