Refactor HttpClient usage

[masojus]

• Consolidate HttpClient usage and common tasks like adding headers into KeenHttpClient.
• Add some flexibility in construction for testing and maybe eventually for client code to customize HTTP settings (like proxy, headers, caching, etc.). We can customize how HttpClients and KeenHttpClients are created/provided and install HTTP handlers in the pipeline.
• Cache HttpClient instances and share them.
• Move some of the HTTP request dispatching code down into KeenHttpClient.
• Test helper class for writing HTTP-related tests and for integration tests in general.
• Fix a few bugs (not_contains filter operator, allow null property value in query filter).
• Fix a few places where Master Key was being required when really the Read Key suffices.
• Demonstrate usage of new test helper classes.
• Get tests working against a real server. Verify changing the Keen URL base authority works so that the custom SSL domains feature can be used via this SDK.
• Change default server url to "https://..."
• Left a few TODOs since there's more work that can be done here, but it might be best to do a larger refactor after deciding which direction to go--we need to consider what's needed for Access Keys, Saved/Cached Queries and Datasets, as well as for supporting http proxies. All of that should get smoothed out before we jump to the 1.0 release.
• Bump version.

[masojus]

Thanks for taking a look @baumatron, I'll get some changes pushed responding to that feedback soon.

Two general notes about these changes: 1) Issue #34 is not fixed by this PR as mentioned in the comments for f70c59b--that was a typo. That change fixes Issues #48 instead. 2) This PR helps us move toward improving Issue #37.