Ipv4 and ipv6 checksum verification


- Added function to calculate checksums for ipv6/ipv4 UDP/TCP
- Added function to verify checksums for ipv6/ipv4 UDP/TCP
under consideration of the constrains of RFC 6935.<br>
- Removed padding from payload, as the available checksum calculation algorithm can cope without.   
- Updated version number to "0.4.2"

Please comment. I will rebase before merging.