Fix the Memcached::Rails fetch method to work when no block is given.
Remove comments for fetch.
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 pull request #127 from joe1chen/memcached-rails-fix-write-unless-exist

Add support for Memcached::Rails write option :unless_exist
Merge pull request #125 from joe1chen/fix-memcached-rails-fetch

Fix the Memcached::Rails fetch method to work when no block is given.