[tharropoulos]

• add SynonymSetItems with retrieve() to list every item in a synonym set
• add SynonymSetItem with retrieve(), upsert() and delete() on /synonym_sets/{name}/items/{id}, URL-encoding both the set name and the item id
• expose both through SynonymSet.items() and SynonymSet.items(itemId), caching one SynonymSetItem per id
• route both Client constructors through one private constructor, which fixes the OkHttpClient constructor leaving synonymSets and curationSets null
• add SynonymSetItemsTest integration tests, skipped on servers below v30
• add ClientTest to check that a custom OkHttpClient handles synonym and curation set calls and that the default constructor still resets the node index

[haydenhoang]

lgtm!