Adding timer functions

The timer is useful and will most likely be used by a lot of applications. Adding functions around the timer will make it easier to use.

Both controlling the timer, and also a delay function which is commonly used.

Proposing these functions;
- `int timer_start(int ticks, int prescaler)`
- `bool timer_is_expired(void)`
- `void timer_stop(void)`
- `void timer_delay(int delay_ms)`