Added UDP ipv6 checksum, checksum calculation and verification functions and test cases.
Updated version number.
Added comment concerning checksum calculation of tunneled packets, of Jumbo packets and of Extension headers
Updated version number from 0.4.1 to 0.4.2
Merge pull request #19 from josemic/ipv4_and_ipv6_checksum_verification

Ipv4 and ipv6 checksum verification