removing scalaz-concurrent dependency

instead of using Promises, this PR reimplements using `scala.concurrent.Future`. see https://github.com/stackmob/newman/issues/52 for more details.

#### Changes in this Pull Request

* all `prepare`, `prepareAsync`, `execute` and `executeUnsafe` calls are gone and replaced with `apply`
* there is no method to automatically block until the response for an `HttpRequest` is complete
* the `apply` method on `HttpRequest` returns `Future[HttpResponse]` now. `Future` is built into the Scala 2.10 library (http://www.scala-lang.org/api/current/index.html#scala.concurrent.Future$)
* `scalaz.effect` and `scalaz.concurrent` have been completely removed as dependencies from Newman. There's no more usage of `IO` and `Promise`

#### What has to Change in your Code

* all `prepare`, `prepareAsync`, `execute` and `executeUnsafe` calls need to become `apply` calls
* the return type of `apply` is `Future`. if you need to block until the response comes back, use [Await](http://www.scala-lang.org/api/current/index.html#scala.concurrent.Await$). I highly recommend refactoring code to avoid blocking if at all possible
* you won't get `scalaz-effect` and `scalaz-concurrent` anymore as a transitive dependency from newman, so you'll have to add it as an explicit dependency. you'll still get `scalaz-core` as a transitive dependency
* there is no more `RichScalaFuture` extension class. Newman converts everything to a Scala future before returning to calling code
* the `ResponseHandlerDSL` and `AsyncResponseHandlerDSL` work on `HttpResponse` and `Future[HttpResponse]`, respectively, and return `Validation[Fail, HttpResponse]` and `Future[Validation[Fail, HttpResponse]]` respectively
* `HttpRequest#concurrentlyWith` and `HttpRequest#andThen` now have different return types, which should make more sense when operating on the new `HttpRequest#apply` method