buffer: add 24-bit operators for buffer module

Some of APIs of `Buffer` what I added listed below:

* `buf.readUIntLE(offset, byteLength, noAssert)`

* `buf.readUIntBE(offset, byteLength, noAssert)`

* `buf.readIntLE(offset, byteLength, noAssert)`

* `buf.readIntBE(offset, byteLength, noAssert)`

* `buf.writeUIntLE(value, offset, byteLength, noAssert)`

* `buf.writeUIntBE(value, offset, byteLength, noAssert)`

* `buf.writeIntLE(value, offset, byteLength, noAssert)`

* `buf.writeIntBE(value, offset, byteLength, noAssert)`

See: http://tools.ietf.org/html/rfc5246#section-7.4
In this spec or other protocols, `uint24` type is often used then these API would simplify related protocols implementation.


The expected value should be wrapped in brackets. e.g.:
```
* `offset` {Number}
* `byteLength` {Number}, Optional, should be `<= 4`, Default: `1`
* `noAssert` {Boolean}, Optional, Default: `false`
* Return: {Number}
```
Looks funny here, but Node's markdown parser handles it differently.