Add setCondition() to Search and Lookup

[ItemSearch](http://docs.aws.amazon.com/AWSECommerceService/latest/DG/ItemSearch.html) and [ItemLookup](http://docs.aws.amazon.com/AWSECommerceService/latest/DG/ItemLookup.html) both have an optional `Condition` parameter that can be set to 'New', 'Used', 'All', etc.

This PR adds explicit setters to `Search` and `Lookup` classes for this parameter.