Pad packet when needed

[p-l-]

This fixes #90.

[p-l-]

Now ready!

[guedou]

Why did you choose 60 ?

According to the page linked in issue #90 this value could be lower. For example, on my laptop sendp("A"*15) works fine while sendp("A"*14) fails. 15 is indeed equal to len(str(Ether())). I wonder if it could be computed dynamically.

[p-l-]

That's not the case for every driver, I have one example where 58 bytes are needed.

I don't think that more than 60 bytes could be needed, since that is the minimum ethernet length, and I think it's fine to have the value set to 60 by default as it can be easily configured.

Also, I have not found a way to find this value dynamically (other than by bisecting, and I don't think that trying multiple values is a good option).

[p-l-]

@guedou I have updated the description on #90 to give more information.

[p-l-]

@guedou agreed! fixed.

[p-l-]

Now fixed and ready to merge!