Add result row for picking the best implementation per layer

This just adds an additional row obtained by cherry-picking the best library per layer (and it right-aligns the timing columns for better readability). Although it's of course the best result, I've made it the last row because of its special role. It can serve a) as a total timing to compare the best single library against, and b) to show how much more can be gained by combining multiple implementations, something that could be done automatically by Theano, for example.

Cheers, and thank you for starting this collection! I'm looking forward to the backward pass (no pun intended), this will probably mix up the order quite a bit.