Mt.Gox improvements

I added code to fetch the timestamp of the last trade. The Mt.Gox ticker API doesn't include this information, so we now do two HTTP requests, the second of which fetches the last trade information.

In the event of the second fetch failing, the timestamp data will be missing in the data returned by the `mtgox_` family of methods, even though it was previously promised by the module's `labels` function.

I've also filled out some more misc labels, added the MtGox module and its tests to the MANIFEST (and the same for MorningstarJP) and added MtGox to the list of modules loaded by default.