Revert to using SKUs for primary ids in Lovefilm -> YouView

As ASINs are unstable, problems are caused by them changing. We're therefore
switching back to using SKUs as the primary identifer, but continuing to
send ASINs in the OtherIdentifiers element, so that it is passed to the
STB app for linking purposes.
Merge pull request #75 from atlasapi/asins-to-skus

Revert to using SKUs for primary ids in Lovefilm -> YouView