SpeedUnit & Unit Test

Added: SpeedUnit for calculations using byte-speed-declarations (powers
of base 1000 instead of powers of base 1024).
Added: A test for both byte and speed units.
Mainboard Changes, Parent XML & Better toString()

Removed: The mainboard no longer is a special part, it's now extending
hardware.
Added: Due to the mainboard change, classes which need a mainboard slot
now have to attach the @NeedsMainboardSlot-annotation.
Added: Several classes no longer need ids for saving the parent object
using XML.
Added: Refined the toString()-methods and added toInfoString() which
delivers a more human-readable string.