sched: consider only positive intervals

It is actually allowed to specify a negative interval, so ignore it in
the sleeptime calculation and in bar_poll_outdated().
sched: execute only timed commands on startup

Add a bar_poll_timed() function meant to be called on startup, to fork
commands with a time interval (not only meant for clicks and signals).

Thus, commands without interval (or defined as 0) won't be executed on
startup. This is useful to simulate a button.

Refs #33
block: introduce special interval value "once"

If "once" is specified for the interval, then the command will be
spawned just once on load. This is useful for one-shot commands, such as
retrieving the hostname or public IP address:

    [ip]
    command=curl -s icanhazip.com
    interval=once

Note that the command will be re-executed on click, which might be
useful for on-demand reload.
block: introduce special interval value "repeat"

If a block interval is specified as "repeat", then its command is
respawned as soon as it terminates. This is useful to repeat blocking
commands, such as watching a cache file:

    [inotify]
    full_text=waiting for writes on /tmp/foo...
    command=touch /tmp/foo; inotifywait -qq -e close_write /tmp/foo; cat /tmp/foo
    interval=repeat
Merge branch 'special-intervals'

* special-intervals:
  block: introduce special interval value "repeat"
  block: introduce special interval value "once"
  sched: execute only timed commands on startup
  sched: consider only positive intervals
block: add a new "label" property

Add a new i3blocks-specific property named "label", used to specify an
optional string to prepend to the full_text after an update. As an
example, this would display something similar to "⌚ 10:57:35":

    [time]
    label=⌚
    command=date +%T
    interval=5
scripts: extract labels

Move labels to the config file instead of letting the scripts print
their own version.
scripts: (battery) avoid utf-8 chars

Refs #36
scripts: (network) take interface from ENV

Make the network script more i3bar-ish by taking the interface from the
BLOCK_INSTANCE environment variable instead of a command line argument.