Split out the Package type into a first class Kubernetes resource (#295)

Split out the Package type into a first class Kubernetes resource. Before this change, packages were implicitly tied to functions.

This wasn't ideal because:
 * Functions will need to share packages
 * A package storage system may be more generally useful than just
   functions (for example, for storing static assets)

This change does the following:
* Updates the fission and tpr types to add a new Package and PackageSpec.  It also creates a PackageRef type, and a FunctionPackageRef type. The PackageRef simply references a package, but the FunctionPackageRef includes the name of a function within the package. This allows us to share packages between different functions.
* Updates fetcher and other components for first-class packages
* Allows customization of fetcher image pull policy in the helm charts

tpr/tpr.go

@@ -95,6 +95,15 @@ func EnsureFissionTPRs(clientset *kubernetes.Clientset) error {
 			},
 			Description: "Message queue triggers for functions",
 		},
+		{
+			ObjectMeta: v1.ObjectMeta{
+				Name: "package.fission.io",
+			},
+			Versions: []v1beta1.APIVersion{
+				{Name: "v1"},
+			},
+			Description: "Packages: archives containing source or binaries for one or more functions",
+		},
 	}
 	for _, tpr := range tprs {
 		err := ensureTPR(clientset, &tpr)