Added string from stdlib

[delneg]

Some functions from "string", although I'm struggling with 'Template' and 'Formatter' classes. (https://github.com/python/typeshed/blob/master/stdlib/string.pyi#L22)
How should they be bind ? Is there any example .. ?
Maybe it's using [] or some other way

[dbrattli]

For string format I was planning to do this:

type System.String with
    [<Emit("$0.format($1...)")>]
    member self.format([<ParamArray>] args: Object[]) = nativeOnly

Is it something similar you need to do for the Formatter class? I've btw added a TestString.fs file, so you can add more unit-tests there for string handling.

[dbrattli]

Some features like named arguments for format is currently blocked by #18