Update Readme about new version of rescu dependency
MtGox api: add support for order/result

Defined the getOrderResult function for getting information about a
completed order. This is an MtGox-specific api, and there is currently
no generic XChange mechanism to access this information across
exchanges. This commit does not add the generic access. However, if the
generic support will be added in the future, it is very likely to be
using the function from this commit anyway.
Until, this function can be used in MtGox-specific way, like MtGoxLag.