Not all attribute classes have all the attributes specified. The
SMPPValueHolder routines for parsing/writing were not tested/executed
and might contain issues. The sm_length/short_message was combined
into a single attribute as it is more like a LV. The question if
the >>readFrom:with: should read the length or not is something that
keeps on coming up. I need to have a more sane way of handling that.
In SMPPv3.4 there are integers with different amounts of bytes.
Let's create a base class that helps with reading them right. Right
now only one byte numbers are supported but that can be changed
in the future.