Improve performance

[magicode]

fix PR #100
Improve performance up 30 times
this benchmark

 
const fs = require('fs');
const filetype = require('.');

var file = fs.readFileSync(__dirname + "/fixture/fixture.png");

var time = 3000;
var starttime = Date.now();
var endtime = starttime + time;
var count = 0;
for(;Date.now()<endtime;count++){
    filetype(file);
}

console.log("%d ops/s" ,~~(count/((Date.now()-starttime)/1000)));

[mifi]

Thanks @magicode. Good idea to check the type, but there are 2 issues:

1. Buffer is already instanceof Uint8array, so no need to check for Buffer
2. Buffer might not exist in a browser environment so that check might give a ReferenceError