Added "offset" to Result.

[de-zakauff]

While working with the SDK, I realized that there is no way to get the offset, so if you don't retain that when passing it to the SDK for querying, the value is lost.

• Added a new "private" field, _offset, which holds the offset value from the result XML, if it is present.
• Added a public getter for the _offset field.
• Updated the constructor so that if the Result XML has an offset attribute, it attempts to parse it to an integer and then set it to the _offset field.
• Updated ResultTest to add tests for the expected offset data.