Add Triangle Sticker

Circle stickers were nixed in #1 due to inefficient space usage. Triangles, on the other hand, [tesselate just fine](http://en.wikipedia.org/wiki/Triangular_tiling).

2.54cm sides would allow some [cool interactions](http://en.wikipedia.org/wiki/Trihexagonal_tiling) with hexagon stickers, but I thought these might be far too small to be of any use. Went with 5.08cm.