Fixes#79
Changes:
1. Renames task `kn-create` to `kn`
2. `kn` task takes a parameter of type array as `ARGS`
3. `kn` task defines a resource of type image as `image`
4. User should reference the `image` as an element of array parameter `ARGS` for e.g. `--image=$(inputs.resources.image.url)`
5. Additional `ClusterRole` for `kn-deployer` account for `revisions` and `routes` resources
6. Adds examples in README.md for creating and updating a Knative Service
7. Adds `kn-image` parameter to task for referencing the kn CLI image to be used, defaults to latest released version of kn image available `gcr.io/knative-releases/github.com/knative/client/cmd/kn`
