WLAN widget

This widget monitors your WLAN's signal strength. To set the device it monitors, use

```lua
    wlan.set_device(dev)
```

The default is "wlan0". Specify "auto" to use the first available WLAN in your system.

To add it to your rc.lua, include

```lua
    local wlan = require("obvious.wlan")
```

in the top of your rc.lua and add `wlan().widget` to your wibox

If you want to use the data gathered by this widget, you can use the function
`wlan.get_data()`. It returns `nil` on failure, otherwise it returns a table with
the field 'link' as its only content. That field contains a number the range of which
depends on the wlan driver and firmware used which describes the signal strength.
