Adding support for purchaseToken field

[bdonahue]

This adds support for the purchaseToken field that is returned as part of the billing data.

Note:
This will work fine if you app has never shipped with the library yet.

Issue to resolve, when the database is reved to version 2, it should probably clear out all transactions and force them to be restored, so the missing purchaseToken field can be fetched. I do not know the library well enough to do that. Hopefully someone that knows this lib better can take this over from here.

[hpique]

Thanks @bdonahue. I won't use your pull request because I already have a version with backward compatibility that I will post sooner than later.

That said, I'll leave it this issue open for a while in case someone else is interested.

[klemensz]

I really need this. See also Issue #95. I wouldn't want to use these changes because I suppose they won't be compatible with the other changes by hpique?