Renamed SnapshotOptions to ClassicSnapshotOptions. Added a similar SnapshotOptions class tailored to the GL-based Static API. Split out ClassicSnapshotTests.

We previously made this initializer failable instead of throwable because of a compiler bug, but the bug appears to have been resolved in Swift 3.

Split out classic overlay tests into a separate test class. Replaced clever but untested URL parsing code in tests with much more straightforward string literals that can be verified easily. Added test fixture images. Verify that NSImage or UIImage can be created from each test fixture image. Switched to mapbox.streets in overlay tests to make fixtures easier to identify. Added configuration tests.

Upgraded example applications to use the new SnapshotOptions initializer. Refined MBSnapshotCamera initializer for Objective-C in the form of a factory method.

Added altitude as an alternative to zoom level in SnapshotCamera, for consistency with MapKit and the Mapbox iOS SDK.

Clarified the relationship between Snapshot and the iOS and macOS SDKs. Added a notice about the general requirement to leave the logo enabled.