Insteon::Message add flag to allow crc16 bytes to be inserted

There is little rhyme or reason as to which packets need to have crc16 versus checksum bytes.  As such, it seems like the proper way to note when a crc16 message should be inserted is by use of a flag.
Merge pull request #7 from krkeegan/insteon_crc16

Insteon::Message add flag to allow crc16 bytes to be inserted