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 check should go at the beginning of the function or else the value will have been written before throwing.