Write logsumexp in a more generic form

[simonbyrne]

This uses Julia's reduction functionality to improve the logsumexp
functionality. It no longer relies on 1-based indexing, is fast
on CuArrays, and there is now a generic fallback for iterable collections.

I've also renamed the 2-arg logsumexp to logaddexp, since Julia
convention is to use different functions for reductions and
reducers (e.g. max vs maximum).

[tpapp]

Very nice work! I think that logaddexp makes a lot of sense. Please consider adding tests for all combinations of Inf, -Inf, NaN. Not because I doubt that your code is correct, but to ensure that all corner cases are covered in case of future changes to this code.

[simonbyrne]

They were pretty exhaustively tested already (I was able to remove some of the branches since we fixed maximum to be NaN absorbing). Are there any other cases you think need adding?