[F# API] More idiomatic API and more ActorSelection methods

I've done few changes for F# API. Most push diffs consider fixing code formatting, so I've decided to enumerate major changes:

### ActorPath parsing
`ActorPath` now has `TryParse` and `TryParseAddress` methods, which are more idiomatic with standard F# API - instead of using ref parameters they return Option type, which may be of type `Some<ActorPath>` / `Some<Address>`, if parsing has succeeded or `None` otherwise.

### ActorSelection
I've created a `select` function, which takes actor path string and returns ActorSelection with anchor set on current ActorCell self. 

Also inspired by [canopy API](http://lefthandedgoat.github.io/canopy/), I've created a tell and ask operators (`<!` and `<?`) overrides, which may operate directly on actor path string instead of actor refs. Since this may cause and operators conflicts and be a controversial feature it requires opening Akka.FSharp.ActorSelection module in order to use.

Examples:
```fsharp
let s: ActorSelection = select "akka://system/user/actor"

open Akka.FSharp.ActorSelection
"akka://system/user/actor" <! Message("hello")
```

### Default Configuration 
Since a `Configuration` module already exists, I've decided to add missing `defaultConfig` method, which returns a default Akka actor system configuration.