Saved/Cached Queries

[masojus]

Fixes Issue #87 by adding support for Saved/Cached Queries.

• This satisfies both the MVP and Stretch goals for that issue.
• Support needed to be added in a back-compat way for parsing raw root JSON Arrays in HTTP responses, which wasn't there before. This affects all clients, including Android, so I'll be sure to test this in the sample apps.
• There was no notion of Master Key usage in the Java SDK outside of Scoped Keys, so that was added. If that's not desirable, we could consider scoping the notion of the Master Key to the SavedQueries interface, but we'd want to be sure that makes sense for future work (Access Keys and Datasets, for starters) which also need Master Key for some functionality.
• There was no notion of an HTTP verb other than POST before this.
• This change does not invent any special representation for responses to PUT requests or the ancillary data returned from .../result endpoints, but rather relies on pre-existing QueryResult to represent the actual query result, and just a Map<String, Object> to represent everything else like the Saved Query definitions. We could look at adding some helpers if it seems frequent that client code wants to access things like run_information and properties related to create/update info.
• Look at individual commit messages for more details.
• I'll follow up with a README/changelog change with new versioning. I was planning to make this be v5.1.0 since it adds a big chunk of functionality.
• I'll open an Issue to fix the __fake_root stuff in a better way that requires a breaking change, but in a vnext_major branch.
• There are a few remaining TODOs in the code. I'm hoping to get some feedback during PR and address those, or else open Issues for them.

[masojus]

I think I've addressed all feedback, either via code changes or by responding here inline or opening an Issue. If this all looks OK, I'll get it merged, and then I'll follow up with a PR that updates the Readme/Changelog and bumps the version to 5.2.0 in preparation for releasing the Maven artifacts.