Use {} for multi-line functions, do/end for procedures

[This article](http://devblog.avdi.org/2011/07/26/the-procedurefunction-block-convention-in-ruby/)
highlights an interesting convention for multi-line blocks, and I think
it makes code clearer, especially when reading a long block. The braces
act as cues -- "Oh, the last expression is going to be used".

I don't really like using `do...end` for one-liners though. Thoughts?