*: AppImageManifest -> ImageManifest
Merge pull request #246 from jonboulle/appimage

*: AppImageManifest -> ImageManifest
appc/ace: update name in validator
appc: fix relative paths in app validator

strings.TrimPrefix was not correct since it resulted in different
behaviour depending on whether the directory passed had a trailing slash
or not (this meant for example that `actool validate foo/` would succeed
but `actool validate foo` would fail)
Go has a function for the correct behaviour in filepath.Rel, so use it.

Also small style updates to keep up with the recent
appmanifest+filesetmanifest --> ImageManifest change
Merge pull request #249 from jonboulle/fix

appc: fix relative paths in app validator
*: Using sha512 instead of sha256
*: Update documentation for sha512, fix missing
*: More sha512 documentation updates
appc: Dont support sha256 for hash at all
Use hash prefixes for rkt run/fetch
appc/spec: updating documentation for prefixes
gofmt
cas: use KeysPrefix diskv method
build: use `godep path` in GOPATH
appc: remove smart quotes
cas: only succeed if there is 1 keyprefix match
cas: documentation standards and punctuation
Don't use godep for build