Battery widget

This widget is a battery monitor. It gets its information from acpi, to be as
uniquely usable as possible. If you click on the widget, additional information
is displayed. Currently, only the first battery is monitored on the widget box,
but information about all other attached batteries is still visible if you click
the widget.

To use it, include it into your rc.lua by inserting this line:
    require("obvious.battery")
into the top of your rc.lua. Then add the widget to your wibox. It's called
    obvious.battery()

If you want to use the data gathered by this widget to create your own, use the
function obvious.battery.get_data(). It returns nil on failure and it returns
a table on success. The table has the following fields:

    state   a string which describes the batteries' state as one element of the
            set ["charged", "full", "discharging", "charging"] (most likely, some
            acpi implementations might output different values)
    charge  a number representing the current battery charge as a number between
            0 and 100
    time    a string describing the time left to full charge or complete discharge
            (the format and whether this field is filled at all depends on the
            acpi implementation)
