appc/spec: clarify AC_APP_NAME
stage1: gpg verify and cache retrieved coreos image
*: use types.Hash to refer to images
Merge pull request #176 from jonboulle/spec

appc/spec: clarify AC_APP_NAME
Merge pull request #175 from jonboulle/hash

*: use types.Hash to refer to images
Merge pull request #174 from vcaputo/cache_pxe_img

stage1: gpg verify and cache retrieved coreos image
tests: add travis-ci testing config for app-container