Add Servo and AngularServo implementation along with docs and tests.
This is a deliberately minimal implementation designed to be added to as
we agree on new extensions (better than making an all-singing,
all-dancing version in which I get things wrong and then wind up making
backward incompatible changes to get it right :)
The prototypes in the docs are rigged to make out the first parameter as
mandatory (as it effectively is); however this does mean you've got to
remember to update the prototype when you modify it in the code! :)
