Refined macros for clock and data manipulation
Created macros for telling elapsed time.

These are useful both for a simple scheduler, and for enforcing clock rate requirements.
In other words, if we only get part of a message, we will notice that it's taking too long
and won't get hung up.
Added main.c and makefile

Most things are still untested (probably will generate many compile errors)
Fixed compile errors

There are two warnings remaining:
(1) That I'm passing a volatle variable via pointer, which discards the attribute
(2) That I'm not reading the variable tmain. This is because I haven't made a scheduler yet.