Convert all finagle-memcached tests to scalatest

Convert all tests in finagle-memcached from specs2 to scalatest.
It's a PR related to issue #290.
I realize this was in the Specs version, but renamed imports are typically put first before normal by-name ones.

    import com.twitter.finagle.memcached.protocol.{
      Error => MemcacheError, ClientError, NonexistentCommand, ServerError}