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've just added a new line between the if statement and the test case:

```scala
if (!Option(System.getProperty("SKIP_FLAKY")).isDefined) 
  test("add and remove") {
```
But I'll change this back. :)