Allow an 'every' timer to be cancelled

If called as:

    my $x = every 5 => sub { };

then the timer can be cancelled sometime later with "undef $x".
Add a section for each function

Describe each function and include an example.
Add new 'after' function

After runs a subroutine only once.

You could do this to, say, make your application stop after some
deadline was reached:

after 3600 => sub { print "Time's up!\n"; app->stop; };