Implement an `Arithmetic` Request supporting `increment` and `decrement` operations

[dkz2]

The goal of this issue is to implement support for increment and decrement operations, specified via the M(I) and M(D) Flags in the Meta Arithmetic (ma) command. This will require us to create a new MemcahedRequest that corresponds to the Meta Arithmetic operation. Additionally this issue will include updating our MemcachedResponseDecoder and MemcachedRequestEncoder to handle request/responses for arithmetic operations. Unit tests for arithmetic request construction and response parsing, as well as a unit and integration test with a Memcached server, and finally adding API support to our MemcachedConnection.

[dkz2]

Completed #29.