Do not use bind in read and write for better performance.

This pull-request remove the use of bind in the `read*` and `write*` method in node.js versions of bops.
This is particularly needed by high-performance node modules, such as the levelup community.

I tested this using this benchmark:
```js
var bops = require('./')
  , buf = new Buffer('hello world')
  , start
  , stop
  , count = 1000000

for (var i = 0; i < count; i++) {
  buf.readUInt8(2)
  bops.readUInt8(buf, 2)
}

start = process.hrtime()

for (var i = 0; i < count; i++) {
  buf.readUInt8(2)
}

stop = process.hrtime()

console.log(stop[0] - start[0], stop[1] - start[1])

start = process.hrtime()

for (var i = 0; i < count; i++) {
  bops.readUInt8(buf, 2)
}

stop = process.hrtime()

console.log(stop[0] - start[0], stop[1] - start[1])
```

Before the patch:
```
Buffer 0 19513627
bops 0 253252617
```

After the patch:
```
Buffer 0 19685158
bops 0 19825163
```