Idiomatic go: use iotas for const declarations

so i feel kinda bad having my first PR be nitpicky, but eh :P

Go makes the [iota](http://golang.org/ref/spec#Iota) identifier available for use in constant declarations. it's generally the preferred way to declare a series of increasing constants (or the flags for a bitfield), unless there's some specific value to be had for programmers in having the values for constants explicitly visible.

i may have been too aggressive with the second commit, where i reorganized the messaging constants into two declarations. it's not clear to me a) why you're specifying in hex and b) what, if any, meaning there is behind the gap in the two sets of numbers; if i missed something on either of those fronts, then iota may not be appropriate here.