Implement `Append` and `Prepend` Support

[dkz2]

The goal of this issue is to implement support for the append and prepend operations. This will require us to add support for the following M(A) / M(P) Flags. We will also need to write unit tests for append and prepend request construction. An integration test where we open a connection with a Memcached server and test the append and prepend functionality will also be included and Finally adding API support to our Connection Actor.