Modified BitOperator to support any BigInteger (basically I just
switched back to using the toByteArray-method of BigInteger, so the
two's complement representation is given back). Put the boilerplate code
for the equals method in every sub class of DataUnit into the DataUnit
class and made the DataUnit class abstract.
Added test coverage in my IDE (not yet in the build). Current tests
cover about 97%, but some of the remaining 3% are because of private
constructors in utility classes and the not yet implemented Ip4Packet