Support PWM on multiple pins

API suggestions encouraged.

The commit linking `Pin`s back to their `Port` makes the printing of pins way uglier. The PWM frequency could be a global instead of a `Port` property if it had to be, because there's only one port with PWM.
`Math.floor(1/(frequency/180000000))` or `(1/(frequency/180000000))|0` to emphasize only integer values are passed in?