Added property to set the template used to build the lookup URL.

I'm not sure if this is the best approach, but the lookup comes up empty if the app is not available in the US Store.

Commit Message
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In order to allow querying for apps that may not be available on the US
Store. For example, the base string for lookup for Dutch-only apps is
"http://itunes.apple.com/nl/lookup?id=%@".