Fix ICMP checksum calc #59
Added icmp rspec
Test rephrasing
typos
Add Travis CI build status badge
Remove old rubies from all_tests
Correct some bugs on IPv6 length calculation.

* Packet#recalc now call ipv6_recalc with arg equals to :all or :ipv6.
* IPv6Header#ipv6_recalc now calls itself with arg=:ipv6_len (instead of
  :len) when called with arg=:all.
Update spec files to make them run with ruby 1.9.3p484, 2.0.0p457 and 2.1.2p95
and with rspec 3.0.0.
Update packetfu.gemspec: rspec must be >= 2.14.1.
Add rspec-its as development dependency in gemspec.
Add IPv6 support for UDPPacket.

UDPPacket#initialize creates an UDP on IPv6 packet when +:on_ipv6 => true+
is passed.
UDPPacket#udp_calc_sum is modified to compute correct checksum for IPv6
packets.
UDPPacket#ipv6? method is added.