Add an A4 size poster for Google Summer of Code 2017, for people to print
and distribute around campus.

This is a minimal update for Google Summer of Code to the English
language GSoC poster from last year. It would be nice to redesign the
poster from scratch but I don't have time right now, others are welcome
to do so. Similarly, a US Letter sized poster would be welcome.

Remove the Polish GSoC 2016 posters, as they are out of date.

Everybody is encouraged to create posters in their local language and add
them here, either using the same design or a design of your own. Note that
the stipend varies on a country by country basis this year: It is likely
best to avoid listing a specific stipend value.