Add Memcached SASL Connection Support

Add SASL connection support

using
```
// using SASL
org.mybatis.caches.memcached.sasl=true
// SASL uername
org.mybatis.caches.memcached.username=${username}
// SASL password
org.mybatis.caches.memcached.password=${password}
```

Reference: https://code.google.com/p/spymemcached/wiki/Examples