Timer rounding

The period register is now rounding to give better accuracy.

Using my sample setup (1.05 mS) I originally came up with a PR of 65
which worked out to being 1.04mS. Using the new rounding math, I get a
PR of 66 which works out to being 1.056mS. While still not 100%
accurate, it is much better.

Fixes #9