Allow for returning output when updating items

[LaurenceGA]

So it's a fairly small change, but it just allows consumers of the library access to the output object of dynamodb's UpdateItem.
For convenience, I've also added an option.UpdateReturnValues function that will allow a consumer to easily set the ReturnValues of the input and hence allow them to easily get back the latest updated item

[nabeken]

Thank you for your contribution. I'll merge this and continue updating the travis-ci to make the tests passed.