Implemented Phil's idea to solve issue #913.

Packets now have an attribute that caches the exact bytes of the packet
when it is captured or read from an str (.dissect()), and this cache is
used when .build() is called, if the cache still exists (e.g., is not None,
e.g., if no field value has been changed).

This should also make operations like writing to a PCAP file captured packets
much faster.

--HG--
branch : Issue #913