Convert legacy GCF events to CloudEvents (GoogleCloudPlatform#285)

This commit adds express middleware to convert legacy GCF requests to the equivalent CloudEvent requests. This enables developers to leverage cloud events in functions that consume events from GCP products that don't yet produce CloudEvents.

Release v1.7.1 (GoogleCloudPlatform#195)

Release v1.7.0 (GoogleCloudPlatform#189)

Release v1.6.0 (GoogleCloudPlatform#151)

Release v1.5.1 (GoogleCloudPlatform#127)

Release v1.5.0 (GoogleCloudPlatform#122)

Release v1.4.0

Release v1.3.2 (GoogleCloudPlatform#94)

Release v1.3.1 (GoogleCloudPlatform#91)

Release v1.3.0 (GoogleCloudPlatform#88)