Amount can just be a uint64

Any thoughts on this?  I figured the struct didn't need to be there and we could simplify things by having it just be a uint64 (actually, if we want negative amounts, maybe int64 is fine).

 amount.go | 39 +++++++++++----------------------------
 client.go | 17 +++++------------
 2 files changed, 16 insertions(+), 40 deletions(-)