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.
the preferred style is to wrap these up in a Context object, and import the context into the test.

a la

```scala
class Ctx {
  val valu1 = mock[Value]
  ...
}

val ctx = new Ctx
import ctx._
...
```

This makes it easier to reason about the tests since we know they aren't sharing any state.