Use buffered io

[chpio]

https://github.com/maidsafe/config_file_handler/blob/630ff159d87062c65b77d1c7918032388b1e122b/src/file_handler.rs#L277-L278

example code from the rust doc:

use std::fs::File;
use std::io::BufReader;
use std::io::prelude::*;

let file = File::open("foo.txt")?;
let mut buf_reader = BufReader::new(file);
let mut contents = String::new();
buf_reader.read_to_string(&mut contents)?;
assert_eq!(contents, "Hello, world!");

maidsafe-archive/crust#865

[chpio]

serde-rs/json#160

[chpio]

this hasn't been fixed yet? i should reopen it then...