Add namespacing to event-type (#137)

* Add namespacing to an event.

This commit has been spliced off of #129

Signed-off-by: Thomas Bouldin <inlined@google.com>

* Update serialization.md to namespace event-type.

Noticed that "namespace" was still in the searialization
docs, even though it was removed from spec.md. Fixed.

Signed-off-by: Thomas Bouldin <inlined@google.com>

* Soften stance on namespacing to SHOULD.

Accept wording suggestions.

This closes issue #32

Signed-off-by: Thomas Bouldin <inlined@google.com>

* Clarify the meaning of the event-type package namespace.

It is explicitly OK for one software organization
to emit event-types of another organizatoin, so
long as they are conforming to the standard set
by the organization in the namespace.

Signed-off-by: Thomas Bouldin <inlined@google.com>

serialization.md

@@ -53,7 +53,7 @@ Example:
   "cloudEventsVersion": "0.1",
   "eventID": "6480da1a-5028-4301-acc3-fbae628207b3",
   "source": "http://example.com/repomanager",
-  "eventType": "created",
+  "eventType": "com.example.repro.create",
   "eventTypeVersion": "v1.5",
   "eventTime": "2018-04-01T23:12:34Z",
   "schemaURL": "https://product.example.com/schema/repo-create",

spec.md

@@ -264,14 +264,15 @@ that contains both context and data).
 
 ### eventType
 * Type: String
-* Description: Type of the event `data`. Producers can specify the format of
-  this, depending on their service. This enables the interpretation of `data`,
-  and can be used for routing, policy and more.
+* Description: Type of occurrence which has happened. Often this
+  property is used for routing, observability, policy enforcement, etc.
 * Constraints:
-  * REQUIRED
-  * MUST be a non-empty string
-* Examples:
-  * customer.created
+   * REQUIRED
+   * MUST be a non-empty string
+   * SHOULD be prefixed with a reverse-DNS name. The prefixed domain dictates
+            the organization which defines the semantics of this event type.
+* Examples
+   * com.github.pull.create
 
 ### eventTypeVersion
 * Type: String