Blanket implementations for usual I/O structures

[nomius10]

My attempt at implementing #20

Python I/O objects usually have a isatty() method:

file = open("foo.txt", "wb")
ret = file.isatty()

This PR mirrors that functionality by adding a trait containing that function, along with blanket implementations for AsRawFD (unix) and AsRawHandle+AsRawSocket (windows). This would allow the following:

use atty::IsATTY;

fn main() {
    let file = std::fs::File::open("foo.txt").unwrap();
    let ret = file.isatty();
}

As far as I can tell, the names I've picked for the trait and method might break naming conventions. I'd appreciate some input on whether the naming should be changed.

Other notes:

• I had to remove nostd