ICMP fix and Packet class should store and provide access to the raw payload in case...

ICMP fix and Packet class should store and provide access to the raw payload in case no packet class is available to parse it.

- Packet to store a copy of the raw payload if no packet class is present to parse it
- Added missing code for ICMP
- Refactored PAcket, ICMP and IPv4 code
- Fixed corrupted flag setting when comparing checksum after deserializing
- Added Junit for IPv4.computeChecksum() and for serilaize/deserialize ICMP message
- Addressed some eclipse and static analysis warnings

Change-Id:If2ae95cbc3462ffcda8662e27efd9de5614872ef
Signed-off-by: Alessandro Boch <aboch@cisco.com>