fix #654: update metro, timer, and delay docs, add all_about_tempo.pd
This adds an all_about_tempo.pd patch to explain the tempo message common to metro, timer, and delay classes.
Pulled from Pd Vanilla-- I added a metro demo in all_about_timer.pd. The only demo in Pd Vanilla is a [delay]
object where a single bang input doesn't exactly give a lot of input into the "tempo" :)