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.


`byteLength` should be coerced, and both coercions should be moved to the top of the function.