Add Sync, Start, and End classes for protocol 7

[synthead]

Add protocol 7 Sync, Start, and End classes.

This one might be a little clunky. Here is a "blank" download from the e-BRAIN software (minus the sync bytes):

07 20 00 00 07 C2 FF
26 90 05 01 44 53 49 20 54 6F 79 73 20 70 72 65 73 65 6E 74 73 2E 2E 2E 65 42 72 61 69 6E 21 00 00 00 00 00 34 07
0D 91 05 01 00 00 00 00 00 00 04 4B 93
05 92 05 62 BD
04 21 D8 C2