appc/spec: Collapse app and fileset manifests

This reworks the app image and fileset concepts to collapse them into a
single manifest, the App Image Manifest. The execution details are now
optional. It also describes a dependency retrieval/matching process
based on labels.
appc/spec: path_whitelist -> pathWhitelist
appc/spec: "app" -> "manifest"
appc/spec: minor cleanup
*: numerous updates to match spec

Changes to schema:
- removing FilesetManifest
- update example manifest to match that in spec
- splitting out App into a dedicated type embedded in AppImageManifest

Changes to tools:
- update aci writer (no more filesets)
- update actool build (no more filesets)
- update actool validate (no more filesets)
- update ace validator to use new types
- update rocket stage0/stage1 to new schema
- update ace app manifests to new spec
- fix a bug in build_aci (double compressing aci)
Merge pull request #241 from jonboulle/spec_app_images

appc/spec: Collapse app and fileset manifests