Endpoint enum

[ajcord]

While working on #14, I discovered that the Endpoint enum in TICL.h has at least one incorrect value (COMP89 should be 0x08). Also, I find it confusing that it only has values for calc->CBL but not the other way around.

I propose using constants more similar to libticalcs' defines along with another enum of just the calculator models, CBL, and PC. Then there can be a mapping function that takes the sender and receiver and outputs the correct machine ID. This would avoid programs assuming incorrect machine IDs.