Add uint64 support to v1

Resolves https://github.com/go-yaml/yaml/issues/49 for v1
It's not clear why there's special support for uint being added. The reason we handle int specially is because it's the default representation for integer literals, and is widely used by default for lengths, etc. What you are adding, though, is a special case that will come into play mainly when int64 overflows, in which case uint is irrelevant. We really want the 64 bits type to solve the problem.