Unicode letters & operators

related issue: https://github.com/fsharp/fsharp/issues/123

examples: 

``` fsharp
let ★ x = x + 5
let (|★) x y = x + y
let summ = ★ 5 |★ 5 // 15
```

``` fsharp
let © = "my copyright notes"
let project = "Heather"
let version = 1
let str = sprintf  "%s v.%d %s" project version ©
```

``` fsharp
let ⌨ msg = Clients.All.addMessage msg
```