Size of int on 64 bit systems is 64 bits in Go 1.1.

This commit changes the use of int -> int32 where needed to make
all the unit tests pass with:

$ go version
go version devel +7cd19e1a734a Wed Mar 27 21:51:07 2013 +0100 darwin/amd64

See http://tip.golang.org/doc/go1.1#int