All packages pre-release version 2.0.0-beta.3

Release of all CloudNative.CloudEvents packages version 2.0.0-beta.2

Changes since 2.0.0-beta.2:

- Addition of HttpRequest.IsCloudEvent/IsCloudEventBatch extension methods
- XML documentation for all members of CloudNative.CloudEvents (work remains for other packages)
- New concept of a "single data type" formatter (documentation) expecting CloudEvent data for one CLR type
- New `CloudEventFormatterAttribute` to allow a type to suggest the formatter to use
- Generic `JsonEventFormatter<T>` types for both JSON implementations

Signed-off-by: Jon Skeet <jonskeet@google.com>