Fix Memcached::Rails increment and decrement to more closely behave like ActiveSupport's memcached implementation. Nil is returned when key is not found or other error is encountered.  Otherwise a non-null return value is converted into an integer.
Merge pull request #126 from joe1chen/fix-memcached-rails-increment-decrement

Fix Memcached::Rails increment and decrement to match ActiveSupport's memcache implementation.