Add option to let limited bandwidth be used by members without vouchers

[nicopace]

Proposed by @nicoechaniz

[luandro]

Changed the name for a bit more clarity on what exactly the feature is.

Basically we need to do add an option in /usr/bin/captive-portal to do some traffic shaping instead of blocking. It seems to be possible with iptables. Maybe @altergui or @gmarcos87 could clarify better if this is the best path.

[nicopace]

I think both the option of limited bandwidth, and limited time should be applied (like will give you 30 mins 3 times per day, and you will have to go through the captive portal each time).
In that way, those that are not part of the CN are encouraged to consume the content you put in the Captive Portal, being news from the village, info about what a CN is and why you should join and how, or where to pay a one time 'don't bother me again' ticket if you don't want to participate with the CN for now.

[patogit]

Also allow traffic limit, as described in #37